Wax ester production by bacteria

Wax ester production by bacteria

244 Wax ester production by bacteria Takeru Ishige, Akio Tani, Yasuyoshi Sakai and Nobuo Kato The enzymological and genetic aspects of microbial met...

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Wax ester production by bacteria Takeru Ishige, Akio Tani, Yasuyoshi Sakai and Nobuo Kato The enzymological and genetic aspects of microbial metabolism of hydrocarbons have been extensively revealed. Such molecular information is useful for understanding the bioremediation of oil spill environments and production of hydrocarbon-specific fine chemicals. Addresses Division of Applied Life Sciences, Graduate School of Agriculture, Kyoto University, Kitashirakawa, Sakyo-ku, Kyoto 606-8502, Japan  e-mail: [email protected]

Current Opinion in Microbiology 2003, 6:244–250 This review comes from a themed issue on Ecology and industrial microbiology Edited by Bernard Witholt and Eugene Rosenberg

amounts from n-alkanes or long-chain alkanols under nitrogen-limited conditions [13]. In several eukaryotes, wax esters have diverse and important biological functions, protecting living cells from desiccation, ultraviolet light and pathogens; they are also a typical energy store of plants. Various types of wax ester from biological sources are widely used in the manufacture of commercial products (i.e. cosmetics, candles, printing inks, lubricants and coating stuffs). In this review, we cover recent progress in the metabolic regulation of bacterial degradation of n-alkanes and synthesis of wax esters from n-alkanes by Acinetobacter spp. We also briefly discuss ester accumulation in some hydrocarbon-degrading Gram-positive bacteria.

1369-5274/03/$ – see front matter ß 2003 Elsevier Science Ltd. All rights reserved.

Metabolic pathway for n-alkanes and its regulation in bacteria

DOI 10.1016/S1369-5274(03)00053-5

The bacterial oxidation of n-alkanes has been extensively characterized using Pseudomonas putida GPo1 (now commonly referred to as Pseudomonas oleovorans GPo1), which uses medium-chain n-alkanes (C5–C12) for growth [15,16,17,18]. The alkane hydroxylase system that is responsible for the total oxidation of an n-alkane to 1-alkanol (RCH3 þ NADH þ Hþ þ O2 ! RCH2OH þ NADþ þ H2O) consists of three components: alkane hydroxylase (AlkB), rubredoxin (AlkG) and rubredoxin reductase (AlkT). AlkB is a non-heme iron integral membrane protein that catalyzes the hydroxylation reaction. AlkG transfers electrons from the NADH-dependent flavoprotein rubredoxin reductase to AlkB. The resultant 1-alkanol is oxidized to 1-alkanoate by a membrane-bound alcohol dehydrogenase (AlkJ) and cytosolic aldehyde dehydrogenase (AlkH). 1-alkanoate is incorporated through b-oxidation via the acyl–CoA synthetase (AlkK) reaction. The gene clusters of alkBFGHJKL and alkST encode these proteins and are located on the OCT plasmid of P. putida GPo1.

Introduction Many microorganisms are able to use petroleum hydrocarbons as growth substrates. Using specific microorganisms, a wide variety of processes for microbial production of chemicals from purified hydrocarbons have been established. During the 1970s, hydrocarbons were commercially attractive resources for bulk biological products, including single-cell protein, amino acids, nucleotides, organic acids and vitamins. More recently, renewable resources, such as agricultural products, are of general interest from the viewpoint of sustainable industrial production, and the use of petroleum hydrocarbons as microbial nutrients for bulk production is declining. However, much attention is being paid to the microbial degradation of hydrocarbons from the viewpoint of bioremediation of areas contaminated with petroleum hydrocarbons [1,2]. Through intensive investigations on microbiological degradation, a wide variety of microbial processes for hydrocarbons have been re-examined. Examples include, unique enzymes related to hydrocarbon degradation [3,4], which can be used in the biotransformation of useful compounds; a variety of bio-emulsifiers [5,6,7], which are produced during microbial assimilation of hydrocarbons; and some cell-reserve materials of hydrocarbon-utilizers [8,9,10,11,12,13]. Bacteria differ in the type of reserve material that they accumulate under restricted conditions; examples include glycogen, polyhydroxyalkanoates, triacylglycerols and polyphosphate [14]. Uniquely, some strains of Acinetobacter produce and accumulate wax esters in enormous Current Opinion in Microbiology 2003, 6:244–250

Other Gram-negative n-alkane degraders belonging to Acinetobacter grow on longer-chain n-alkanes. Although the reactions for alkane oxidation of Acinetobacter sp. strain ADP1, which uses n-alkanes C12–C18, are principally analogous to those of P. putida GPo1, the organization of the genes involved in n-alkane oxidation is different [19–21]. In Acinetobacter sp. strain ADP1, five chromosomal genes (alkM, rubA, rubB, alkR and xcpR) in at least three different loci are required for n-alkane degradation. Acinetobacter sp. strain M-1 is characterized by its ability to use much longer-chain n-alkanes (C20–C44), and can degrade n-alkanes up to C60 when grown on a ‘paraffin www.current-opinion.com

Bacterial wax esters Ishige et al. 245

wax’ mixture. This strain possesses two chromosomal genes (alkMa and alkMb) that encode alkane hydroxylases, which are located in two different loci on the chromosomal DNA [22]. AlkMa is highly similar to AlkM of Acinetobacter sp. ADP1 (84% amino acid sequence identity), whereas AlkMa and AlkMb show 61% amino acid sequence identity with each other [13]. The amino acid sequence identity between AlkMRa and AlkMRb, which are transcriptional regulators for AlkMa and AlkMb, respectively, is only 5% (i.e. they are only very distantly related). This also implies that alkMa and alkMb might be regulated differently in this strain. From the results of genetic and biochemical studies, it is concluded that alkMa expression is induced by very-long-chain alkanes (>C22), which are solid at ambient temperatures, and alkMb expression is induced preferentially by shorter-chain n-alkanes. However, the genes for rubredoxin (RubA) and rubredoxin reductase (RubB) are encoded on the rubAB operon in a different locus on the chromosomal DNA from those of alkMa and alkMb. From these facts, we propose a model for the regulation of the alkane hydroxylase system of Acinetobacter sp. strain M-1 (Figure 1). According to this model, the organism controls alkane hydroxylase activity in response to the chain-length of

the substrate, by switching the alkane hydroxylase component, AlkMa or AlkMb, without changing other components of the other complex, RubA and RubB. Several types of pyridine-nucleotide-dependent dehydrogenases for alkanol and alkanal oxidation have been found in the cytosolic fractions of n-alkane-grown Acinetobacter spp. In strain M-1, the genes for NADPþ-dependent alcohol dehydrogenase (AlrA) [23] and NADþdependent aldehyde dehydrogenase (AldI) [24] are expressed inductively with n-alkanes. These enzymes have important roles in wax ester synthesis, rather than in the principal oxidation of n-alkanes. In addition, Acinetobacter sp. M-1 possesses two genes encoding acyl–CoA dehydrogenases, acdA and acdB, which are arranged in tandem on the chromosomal DNA [25]. AcdA and AcdB are active toward medium-chain and long-chain acyl– CoAs, respectively. Characterization of n-alkane metabolism in Acinetobacter spp has revealed a parallel pathway, as well as enzymes with overlapping specificities in a single pathway. Such physiological complexity seems to be due to the fact that the n-alkane oxidation pathway is used for different purposes, such as growth on n-alkanes and synthesis of wax esters as cell reserves [23,24].

Figure 1

Very-long-chain n-alkane

Long-chain n-alkane

Outer membrane Periplasm Cytoplasmic membrane

AlkMb

AlkMa RubA

RubA

Cytoplasm

RubB

RubB

Inducible expression

Inducible expression Constitutive expression

gshR

alkMa

alkRa

lpxA lysS

ahpC rubA

alkMb

alkRb

mdmC

rubB Current Opinion in Microbiology

A model for the regulation of the alkane hydroxylase system of Acinetobacter sp. strain M-1. alkMa and alkMb encode alkane hydroxylase in Acinetobacter sp. strain M-1. alkMa and alkMb expression is induced by very-long-chain n-alkenes (>C22) and long-chain n-alkanes (
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246 Ecology and industrial microbiology

Apart from their physiological significance, some enzymes have attractive features as biological catalysts. AlrA, which is a thermostable alcohol dehydrogenase, exhibits notably high reductive activity toward medium-chain alkanals; this substrate specificity being evidently different from those of other enzymes (for example, the specific

activity for 1-hexadecanal is about 100 times higher than that of horse liver alcohol dehydrogenase) [23]. AldI is also a thermostable enzyme, whose activity becomes higher with increasing substrate carbon-chain length, the most preferable substrate tested being 1-tetradecanal, which is the longest alkanal commercially available [24].

Figure 2

(a)

(b)

Wax ester

200 nm (c)

Wax ester

200 nm (d)

200 nm 200 nm (e)

(f) Disk 1

Disk

Disk 2

200 nm

200 nm Current Opinion in Microbiology

Electron microscopy of accumulated wax ester in Acinetobacter sp. strain M-1. Thin sections of cells were grown on (a) yeast extract-tryptone medium (2  YT medium) and (b) after a shift to nitrogen-limited medium containing 0.5% n-hexadecane incubated for 2 h (b,c) and 10 h (d) at 308C. Cells in (b and c) were incubated under the same conditions but sliced differently. (e,f) Images were obtained by the quick-freezing replica method of cells prepared under the same conditions as in (c). When cells grown on 2 x YT medium (a) were transferred into the medium containing 0.5% n-hexadecane (b), characteristic intracellular inclusion bodies containing wax esters were formed. The number of inclusion bodies increased during the incubation period (b,d). The inclusion bodies of wax esters were disk-shaped, had a smooth surface and grew to almost the same diameter as the cells (c,e,f). Current Opinion in Microbiology 2003, 6:244–250

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Bacterial wax esters Ishige et al. 247

High GþC- and mycolic-acid-containing Gram-positive bacteria, such as Rhodococcus, Mycobacterium, Corynebacterium, Gordonia, Nocardia are known to degrade n-alkanes with a wide range of carbon-chain lengths [26–29]. This has been confirmed by molecular screening for alk-like genes in the microbial populations found in oil-contaminated environments [30,31]. Interestingly, two newly isolated strains of Rhodococcus contain at least four alkane hydroxylase homologs (alkB1, alkB2, alkB3 and alkB4), the function of one of which (alkB2) has been confirmed [18,32]. Such Gram-positive bacteria are also of considerable interest with regard to their hydrocarbon-specific activities applicable to biotechnological processes.

bated with 3% n-hexadecane for 10 h at 308C. The produced wax ester comprised hexadecyl hexadecanoate (98%) and hexadecyl myristate (2%). Wax ester production from various n-alkanes by Acinetobacter sp. strain M-1 is shown in Figure 3. When n-alkanes with longer-carbon chains were used, wax esters containing alkanoates with shorter-carbon chains than that of the substrate increase, whereas the alkanol moieties were principally the same as those of the substrate n-alkanes in chain length. This implies that the chain length of some acyl–CoAs, which are shortened through the b-oxidation cycle, are also used for the final step of wax ester synthesis.

Wax ester production by Acinetobacter spp

The accumulation of a large amount of wax esters derived from n-alkanes could be a notable feature of strain M-1; other strains of Acinetobacter might accumulate wax esters derived from n-alkyl alcohols. This can be explained by the fact that the alkanol supply from n-alkanes is higher in strain M-1 than in other strains. The proposed pathway for wax ester synthesis by Acinetobacter spp is shown in Figure 4. The final step of wax ester synthesis is catalyzed by an acyl–CoA:alcohol transferase, which was recently characterized as a novel bifunctional wax ester synthase/acyl–CoA:diacylglycerol acyltransferase of A. carcoaceticus ADP1 by Kalscheuer and Steinbu¨ chel [33]. In Acinetobacter sp. strain M-1, there are two possible routes for supplying alcohol to the final step. One route is conversion of acyl–CoA to alcohol, through the sequential reactions of acyl–CoA reductase and aldehyde reductase; the latter is possibly NADPþ-dependent alcohol dehydrogenase (ArlA) [23].

Several species of Acinetobacter are known to accumulate wax esters when they are transferred to a nitrogenlimited medium containing n-alkanes or 1-alkanols [13]. Thin sections of Acinetobacter sp. strain M-1 cells reveal electron-transparent intracellular inclusion bodies (Figure 2), which consist of wax esters. Quickfreezing replica microscopy more clearly shows the structure of the inclusion bodies, which are disk-shaped, have a smooth surface, and grow to almost the same diameter as the cells (Figure 2). No intracytoplasmic membrane structures or limiting membranes surrounding these inclusions have been observed. The wax ester accumulation proceeds with the sequential formation of disks that are 30–50 nm in depth, (i.e. the completion of one disk leads to that of another). Wax esters were accumulated optimally up to 0.17 g/g of cells (dry weight) when cells (1.85  1010 cells/ml) were incu-

Figure 3

Substrate n-alkane

Composition of wax ester (R1–CO–OR2)

Wax ester accumulated (mg/mg cell)

C16–C16

C16

0.17 C16–C14

C18–C18

C18

C18–C16

0.16 C18–C14

C20

C22

C20–C20

C22–C22

C20–C18 C20–C16

C22–C20

0.13

0.11 C22–C18 C22–C16

C24

C24–C24

0.09

C24–C22 C24–C20 C24–C18 Wax ester composition and production from various n-alkanes by Acinetobacter sp. strain M-1. The cells grown on yeast extract-tryptone medium were transferred to nitrogen-limiting media containing n-alkanes (C16–C24) and then incubated at 308C for 10 h. www.current-opinion.com

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248 Ecology and industrial microbiology

Figure 4

n-alkane (a) alkanol

OH

(b) alkanal (c) alkanoate

β-oxidation Cell constituents

CHO

COOH

(d)

Acyl–CoA O

(e)

O CHO

(f) alkanol

C

(g)

alkanal

Wax ester

OH

The proposed carbon flow from n-alkanes to wax esters in Acinetobacter spp (a) alkane monooxygenase, (b) alcohol dehydrogenase, (c) aldehyde dehydrogenase, (d) acyl–CoA synthetase, (e) acyl–CoA reductase, (f) aldehyde reductase (alcohol dehydrogenase) and (g) acyl–CoA:alcohol transferase.

The second route is direct introduction from 1-alkanols or 1-alkanals, each of which is an intermediate of n-alkane oxidation. Alternatively, the alkanoate moiety of wax esters are exclusively derived from acyl–CoA. In acyl–CoA reductase deficient mutants (e.g. acrM-KO of strain M-1 [13] and Wpw15 of strain ADP1 [34]), no wax ester was formed from 1-hexadecanoate. Using the acrM-KO strain, the alkanol and alkanoate compositions of wax ester could be controlled precisely by alteration of the substrate, as the alkanol moiety is fixed to the substrate n-alkanes and a desirable carboxylic acid moiety is directly introduced from the substrate added.

als, growth conditions and the mutant strain. Acinetobacter sp. strain M-1 could be a good candidate for this purpose, judging from its ability to use a broad range of n-alkanes. The accumulation of wax esters, as well as triacylglycerols, by Gram-positive bacteria occurs specifically during the degradation of hydrocarbons under nitrogen-limited conditions. Such restricted conditions normally predominate in a natural environment. Understanding metabolic regulation under restricted conditions could be important not only for wax ester production but also for bioremediation.

References and recommended reading Wax ester of Rhodococcus sp. Rhodococcus opacus PD630 accumulates a unique wax ester from phenyldecane during cultivation under nitrogenlimited conditions [8]. The wax ester, phenyldecyl phenyldecanoate, is assumed to be produced through the condensation of the oxidation intermediates phenyldecanol and penyldecanoic acid [35]. The organism also accumulates novel triacylglycerols, one fatty acid of which is replaced by a phenylcarboxylic acid derived from the growth substrate, phenyldecane [36]. In this context, several Gram-positive bacteria are also able to produce and accumulate triacylglycerols from different carbon sources, including hydrocarbons [37–39].

Conclusions The microbial production of wax esters has advantages over other biological processes, as the wax ester composition can be controlled by the choice of starting materiCurrent Opinion in Microbiology 2003, 6:244–250

Papers of particular interest, published within the annual period of review, have been highlighted as:  of special interest  of outstanding interest 1.

Hughes BJ, Neele CN, Ward CH: Bioremediation. In Encyclopedia of Microbiology, vol 1. Edited by Lederberg J. New York: Academic Press; 2000:587-610.

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Li Z, van Beilen JB, Duetz WA, Schmid A, de Raadt A, Griengl H, Witholt B: Oxidative biotransformations using oxygenases. Curr Opin Chem Biol 2002, 6:136-144. From a biotechnological perspective, this review summarizes the usefulness of several oxygenases, including ones from hydrocarbon-degrading microorganisms, for reoselective and stereoselective production of useful compounds. 4.

Mathys RG, Schmid A, Witholt B: Integrated two-liquid phase bioconversion and product-recovery processes for the oxidation of alkanes: process design and economic evaluation. Biotechnol Bioeng 1999, 64:459-477. www.current-opinion.com

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Toren A, Navon-Venezia S, Ron EZ, Rosenberg E: Emulsifying activities of purified Alasan proteins from Acinetobacter radioresistens KA53. Appl Environ Microbiol 2001, 67:1102-1106.

6. 

Toren A, Orr E, Paitan Y, Ron EZ, Rosenberg E: The active component of the bioemulsifier alasan from Acinetobacter radioresistens KA53 is an OmpA-like protein. J Bacteriol 2002, 184:165-170. The authors report on the cloning and overexpression of the gene responsible for synthesis of a protein component of bioemulsan (alasan) from Acinetobacter radioresistens KA53. The results will contribute to understanding of the molecular function of alasan. 7.

Toren A, Segal G, Ron EZ, Rosenberg E: Structure–function studies of the recombinant protein bioemulsifier AlnA. Environ Microbiol 2002, 4:257-261.

8.

Alvarez HM, Kalscheuer R, Steinbuchel A: Accumulation and mobilization of storage lipids by Rhodococcus opacus PD630 and Rhodococcus ruber NCIMB 40126. Appl Microbiol Biotechnol 2000, 54:218-223.

9.

Durner R, Zinn M, Witholt B, Egli T: Accumulation of poly[(R)-3hydroxyalkanoates] in Pseudomonas oleovorans during growth in batch and chemostat culture with different carbon sources. Biotechnol Bioeng 2001, 72:278-288.

10. Kessler B, Witholt B: Factors involved in the regulatory network  of polyhydroxyalkanoate metabolism. J Bacteriol 2001, 86:97-104. Polyhydroxyalkanoates (PHAs) are bacterial storage materials, and promising biotechnological products from a variety of carbon sources including n-alkanes. This paper points out the diversity of regulatory mechanisms involved in PHA metabolism. 11. Jung K, Hazenberg W, Prieto M, Witholt B: Two-stage continuous process development for the production of medium-chainlength poly(3-hydroxyalkanoates). Biotechnol Bioeng 2001, 72:19-24. 12. Spiekermann P, Rehm BH, Kalscheuer R, Baumeister D, Steinbuchel A: A sensitive, viable-colony staining method using Nile red for direct screening of bacteria that accumulate polyhydroxyalkanoic acids and other lipid storage compounds. Arch Microbiol 1999, 171:73-80. 13. Ishige T, Tani A, Takabe K, Kawasaki K, Sakai Y, Kato N: Wax ester  production from n-alkanes by Acinetobacter sp. strain M-1: ultrastructure of cellular inclusions and role of acyl coenzyme A reductase. Appl Environ Microbiol 2002, 68:1192-1195. The authors report on wax ester accumulation from n-alkanes by Acinetobacter sp. strain M-1. They reveal the fine structure of the inclusion bodies containing wax ester and show that the composition of wax ester might be controlled precisely by alteration of the substrates in the acrM disruptant strains. 14. Finnarty WR: Biopolymers, production and uses of. In Encyclopedia of Microbiology, vol 1. Edited by Lederberg J. NY: Academic Press; 2000. 15. Staijen IE, Van Beilen JB, Witholt B: Expression, stability and performance of the three-component alkane mono-oxygenase of Pseudomonas oleovorans in Escherichia coli. Eur J Biochem 2000, 267:1957-1965. 16. van Beilen JB, Panke S, Lucchini S, Franchini AG, Rothlisberger M,  Witholt B: Analysis of Pseudomonas putida alkane-degradation gene clusters and flanking insertion sequences: evolution and regulation of the alk genes. Microbiology 2001, 147:1621-1630. This is a relevant paper for understanding the regulation of the n-alkane oxidation pathway in Pseudomonas putida, and also describes the functions of genes flanking the alk gene clusters located on the OCT plasmid. 17. van Beilen JB, Neuenschwander M, Smits TH, Roth C, Balada SB, Witholt B: Rubredoxins involved in alkane oxidation. J Bacteriol 2002, 184:1722-1732. 18. Smits TH, Balada SB, Witholt B, van Beilen JB: Functional analysis of alkane hydroxylases from Gram-negative and Gram-positive bacteria. J Bacteriol 2002, 184:1733-1742. 19. Ratajczak A, Geissdorfer W, Hillen W: Expression of alkane hydroxylase from Acinetobacter sp. Strain ADP1 is induced by a broad range of n-alkanes and requires the transcriptional activator AlkR. J Bacteriol 1998, 180:5822-5827. www.current-opinion.com

20. Ratajczak A, Geissdorfer W, Hillen W: Alkane hydroxylase from Acinetobacter sp. strain ADP1 is encoded by alkM and belongs to a new family of bacterial integral-membrane hydrocarbon hydroxylases. Appl Environ Microbiol 1998, 64:1175-1179. 21. Geissdorfer W, Kok RG, Ratajczak A, Hellingwerf KJ, Hillen W: The genes rubA and rubB for alkane degradation in Acinetobacter sp. strain ADP1 are in an operon with estB, encoding an esterase, and oxyR. J Bacteriol 1999, 181:4292-4298. 22. Tani A, Ishige T, Sakai Y, Kato N: Gene structures and regulation  of the alkane hydroxylase complex in Acinetobacter sp. strain M-1. J Bacteriol 2001, 183:1819-1823. The authors report that Acinetobacter sp. strain M-1 has two genes encoding alkane hydroxylase that are differentially induced in response to the chain length of n-alkanes. 23. Tani A, Sakai Y, Ishige T, Kato N: Thermostable NADPþdependent medium-chain alcohol dehydrogenase from Acinetobacter sp. strain M-1: purification and characterization and gene expression in Escherichia coli. Appl Environ Microbiol 2000, 66:5231-5235. 24. Ishige T, Tani A, Sakai Y, Kato N: Long-chain aldehyde dehydrogenase that participates in n-alkane utilization and wax ester synthesis in Acinetobacter sp. strain M-1. Appl Environ Microbiol 2000, 66:3481-3486. 25. Tani A, Ishige T, Sakai Y, Kato N: Two acyl-CoA dehydrogenases of Acinetobacter sp. strain M-1 that uses very long-chain n-alkanes. J Biosci Biotechnol 2002, 94:326-329. 26. Smith TJ, Lloyd JS, Gallagher SC, Fosdike WL, Murrell JC, Dalton H: Heterologous expression of alkene monooxygenase from Rhodococcus rhodochrous B-276. Eur J Biochem 1999, 260:446-452. 27. Hamamura N, Arp DJ: Isolation and characterization of alkaneutilizing Nocardioides sp. strain CF8. FEMS Microbiol Lett 2000, 186:21-26. 28. Hamamura N, Yeager CM, Arp DJ: Two distinct monooxygenases for alkane oxidation in Nocardioides sp. strain CF8. Appl Environ Microbiol 2001, 67:4992-4998. 29. van Beilen JB, Smits TH, Whyte LG, Schorcht S, Rothlisberger M, Plaggemeier T, Engesser KH, Witholt B: Alkane hydroxylase homologues in Gram-positive strains. Environ Microbiol 2002, 4:676-682. 30. Smits TH, Rothlisberger M, Witholt B, van Beilen JB: Molecular screening for alkane hydroxylase genes in Gram-negative and Gram-positive strains. Environ Microbiol 1999, 1:307-317. 31. Juck D, Charles T, Whyte LG, Greer CW: Polyphasic microbial community analysis of petroleum hydrocarbon-contaminated soils from two northern Canadian communities. FEMS Microbiol Ecol 2000, 33:241-249. 32. Whyte LG, Smits TH, Labbe D, Witholt B, Greer CW, Van Beilen JB:  Gene cloning and characterization of multiple alkane hydroxylase systems in Rhodococcus strains Q15 and NRRLB16531. Appl Environ Microbiol 2002, 68:5933-5942. Much less is known about the alkane-degrading systems of Grampositive bacteria. This paper reports that two strains of genus Rhodococcus have multiple gene clusters for alkane hydroxylase systems, and the enzymatic functions of some of the gene products are revealed. 33. Kalscheuer R, Steinbu¨ chel A: A novel bifunctional wax ester  synthase/acyl-CoA:diacylglycerol acyltransferase mediates wax ester and triacylglycerol biosynthesis in Acinetobacter calcoaceticus ADP1. J Biol Chem 2002, 26:26. This is the first report of the bacterial enzyme that catalyzes the synthesis of wax ester from an acyl–CoA and an alkanol. This enzyme has also acyl– CoA:diacylglycerol acyltransferase activity, which is distinct from wax ester synthase from jojoba. 34. Reiser S, Somerville C: Isolation of mutants of Acinetobacter calcoaceticus deficient in wax ester synthesis and complementation of one mutation with a gene encoding a fatty acyl coenzyme A reductase. J Bacteriol 1997, 179:2969-2975. 35. Alvarez HM, Luftmann H, Silva RA, Cesari AC, Viale A, Waltermann M, Steinbuchel A: Identification of phenyldecanoic acid as a constituent of triacylglycerols and wax ester produced by Current Opinion in Microbiology 2003, 6:244–250

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Rhodococcus opacus PD630. Microbiology 2002, 148:1407-1412. 36. Alvarez HM, Steinbuchel A: Triacylglycerols in prokaryotic microorganisms. Appl Microbiol Biotechnol 2002, 60:367-376. 37. Garton NJ, Christensen H, Minnikin DE, Adegbola RA, Barer MR: Intracellular lipophilic inclusions of mycobacteria in vitro and in sputum. Microbiology 2002, 148:2951-2958.

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38. Kalscheuer R, Waltermann M, Alvarez M, Steinbuchel A: Preparative isolation of lipid inclusions from Rhodococcus opacus and Rhodococcus ruber and identification of granuleassociated proteins. Arch Microbiol 2001, 177:20-28. 39. van ver Meer MT, Schouten S, Hanada S, Hopmans EC, Damste JS, Ward DM: Alkane-1,2-diol-based glycosides and fatty glycosides and wax esters in Roseiflexus castenholzii and hot spring microbial mats. Arch Microbiol 2002, 178:229-237.

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