Geranic acid formation, an initial reaction of anaerobic monoterpene metabolism in denitrifying Alcaligenes defragrans

Double-duty isomerases: a case study of isomerization-coupled enzymatic catalysis

Enzymes can usually be unambiguously assigned to one of seven classes specifying the basic chemistry of their catalyzed reactions. Less frequently, two or more reaction classes are catalyzed by a single enzyme within one active site. Two examples are an isomerohydrolase and an isomero-oxygenase that catalyze isomerization-coupled reactions crucial for production of vision-supporting 11-cis-retinoids. In these enzymes, isomerization is obligately paired and mechanistically intertwined with a second reaction class. A handful of other enzymes carrying out similarly coupled isomerization reactions have been described, some of which have been subjected to detailed structure-function analyses. Herein we review these rarefied enzymes, focusing on the mechanistic and structural basis of their reaction coupling with the goal of revealing catalytic commonalities.

Terpenes modulate bacterial and fungal growth and sorghum rhizobiome communities

Terpenes are among the oldest and largest class of plant-specialized bioproducts that are known to affect plant development, adaptation, and biological interactions. While their biosynthesis, evolution, and function in aboveground interactions with insects and individual microbial species are well studied, how different terpenes impact plant microbiomes belowground is much less understood. Here we designed an experiment to assess how belowground exogenous applications of monoterpenes (1,8-cineole and linalool) and a sesquiterpene (nerolidol) delivered through an artificial root system impacted its belowground bacterial and fungal microbiome. We found that the terpene applications had significant and variable impacts on bacterial and fungal communities, depending on terpene class and concentration; however, these impacts were localized to the artificial root system and the fungal rhizosphere. We complemented this experiment with pure culture bioassays on responsive bacteria and fungi isolated from the sorghum rhizobiome. Overall, higher concentrations (200 µM) of nerolidol were inhibitory to Ferrovibrium and tested Firmicutes. While fungal isolates of Penicillium and Periconia were also more inhibited by higher concentrations (200 µM) of nerolidol, Clonostachys was enhanced at this higher level and together with Humicola was inhibited by the lower concentration tested (100 µM). On the other hand, 1,8-cineole had an inhibitory effect on Orbilia at both tested concentrations but had a promotive effect at 100 µM on Penicillium and Periconia. Similarly, linalool at 100 µM had significant growth promotion in Mortierella, but an inhibitory effect for Orbilia. Together, these results highlight the variable direct effects of terpenes on single microbial isolates and demonstrate the complexity of microbe-terpene interactions in the rhizobiome. IMPORTANCE Terpenes represent one of the largest and oldest classes of plant-specialized metabolism, but their role in the belowground microbiome is poorly understood. Here, we used a "rhizobox" mesocosm experimental set-up to supply different concentrations and classes of terpenes into the soil compartment with growing sorghum for 1 month to assess how these terpenes affect sorghum bacterial and fungal rhizobiome communities. Changes in bacterial and fungal communities between treatments belowground were characterized, followed by bioassays screening on bacterial and fungal isolates from the sorghum rhizosphere against terpenes to validate direct microbial responses. We found that microbial growth stimulatory and inhibitory effects were localized, terpene specific, dose dependent, and transient in time. This work paves the way for engineering terpene metabolisms in plant microbiomes for improved sustainable agriculture and bioenergy crop production.

Potential enzymes involved in beer monoterpenoids transformation: structures, functions and challenges

Monoterpenes are important flavor and fragrance compounds in food. In beer, the monoterpenes mainly come from hops added during boiling process. Biotransformations of monoterpene which occurred during fermentation conferred beer with various kinds of aroma profiles, which can be mainly attributed to the contribution of enzymes in yeast. However, there are few reports on the identification and characterization of these enzymes in yeast. Illustrating the structure and functions of key enzymes related to transformations will broaden their potential applications in beer or other foodstuffs. Monoterpenoids including terpene hydrocarbons (limonene, myrcene, and pinene) and terpene alcohol (linalool, geraniol, nerol, and citronellol) gave the beer flower-like or fruit-like aroma. The biotransformation of monoterpenes and monoterpene alcohols in bacteria and yeast, and potential enzymes related to the transformation of them are reviewed here. Enzymes primarily are dehydrogenases including linalool dehydrogenase/isomerase, geraniol/geranial dehydrogenase, nerol dehydrogenase and citronellol dehydrogenase. Most of them are substrate-specific or substrate-specific after modifications by biotechnology methods, and part of them have been expressed in E. coli, while the purification and catalytic rate is very low. Efforts should be made to acquire abundant enzymes, and to fabricate enzyme-expressing yeast, which can be further applied in beer fermentation system.highlightsMonoterpenoids contributed to the flavor of food, especially beer.Transformation of monoterpenoids occurred during fermentation.Various kinds of enzymes are involved in the transformation of monoterpenoids in bacteria, yeast, etc.Crystal structures of these enzymes have been partially resolved.Few enzymes are further applied in food system to obtain abundant flavor.

Limonene dehydrogenase hydroxylates the allylic methyl group of cyclic monoterpenes in the anaerobic terpene degradation by Castellaniella defragrans

The enzymatic functionalization of hydrocarbons is a central step in the global carbon cycle initiating the mineralization of methane, isoprenes, and monoterpenes, the most abundant biologically produced hydrocarbons. Also, terpene-modifying enzymes have found many applications in the energy-economic biotechnological production of fine chemicals. Here, we describe a limonene dehydrogenase that was purified from the facultatively anaerobic betaproteobacterium Castellaniella defragrans 65Phen grown on monoterpenes under denitrifying conditions in the absence of molecular oxygen. The purified limonene:ferrocenium oxidoreductase activity hydroxylated the methyl group of limonene (1-methyl-4-(1-methylethenyl)-cyclohex-1-ene) yielding perillyl alcohol ([4-(prop-1-en-2-yl)cyclohex-1-en-1-yl]methanol). The enzyme had a DTT:perillyl alcohol oxidoreductase activity yielding limonene. Mass spectrometry and molecular size determinations revealed a heterodimeric enzyme comprising CtmA and CtmB. Recently, the two proteins had been identified by transposon mutagenesis and proteomics as part of the cyclic terpene metabolism (ctm) in C. defragrans and are annotated as FAD-dependent oxidoreductases of the protein domain family phytoene dehydrogenases and related proteins (COG1233). CtmAB is the first heterodimeric enzyme in this protein superfamily. Flavins in the purified CtmAB are oxidized by ferrocenium and are reduced by limonene. Heterologous expression of CtmA, CtmB, and CtmAB in Escherichia coli demonstrated that limonene dehydrogenase activity required both subunits, each carrying a flavin cofactor. Native CtmAB oxidized a wide range of monocyclic monoterpenes containing the allylic methyl group motif (1-methyl-cyclohex-1-ene). In conclusion, we have identified CtmAB as a hydroxylating limonene dehydrogenase and the first heteromer in a family of FAD-dependent dehydrogenases acting on allylic methylene or methyl CH-bonds. We suggest placing in Enzyme Nomenclature as new entry EC 1.17.99.8.

Protonation state and fine structure of the active site determine the reactivity of dehydratase: hydration and isomerization of β-myrcene catalyzed by linalool dehydratase/isomerase from Castellaniella defragrans

Linalool dehydratase/isomerase (LinD) from Castellaniella defragrans is a bifunctional enzyme that catalyzes the hydration of β-myrcene to (S)-linalool and isomerization of (S)-linalool to geraniol.

Effects of monoterpenes on ion channels of excitable cells

Monoterpenes are a structurally diverse group of phytochemicals and a major constituent of plant-derived 'essential oils'. Monoterpenes such as menthol, carvacrol, and eugenol have been utilized for therapeutical purposes and food additives for centuries and have been reported to have anti-inflammatory, antioxidant and analgesic actions. In recent years there has been increasing interest in understanding the pharmacological actions of these molecules. There is evidence indicating that monoterpenes can modulate the functional properties of several types of voltage and ligand-gated ion channels, suggesting that some of their pharmacological actions may be mediated by modulations of ion channel function. In this report, we review the literature concerning the interaction of monoterpenes with various ion channels.

Deciphering the genome repertoire of Pseudomonas sp. M1 toward β-myrcene biotransformation

Pseudomonas sp. M1 is able to mineralize several unusual substrates of natural and xenobiotic origin, contributing to its competence to thrive in different ecological niches. In this work, the genome of M1 strain was resequenced by Illumina MiSeq to refine the quality of a published draft by resolving the majority of repeat-rich regions. In silico genome analysis led to the prediction of metabolic pathways involved in biotransformation of several unusual substrates (e.g., plant-derived volatiles), providing clues on the genomic complement required for such biodegrading/biotransformation functionalities. Pseudomonas sp. M1 exhibits a particular sensory and biotransformation/biocatalysis potential toward β-myrcene, a terpene vastly used in industries worldwide. Therefore, the genomic responsiveness of M1 strain toward β-myrcene was investigated, using an RNA sequencing approach. M1 cells challenged with β-myrcene(compared with cells grown in lactate) undergo an extensive alteration of the transcriptome expression profile, including 1,873 genes evidencing at least 1.5-fold of altered expression (627 upregulated and 1,246 downregulated), toward β-myrcene-imposed molecular adaptation and cellular specialization. A thorough data analysis identified a novel 28-kb genomic island, whose expression was strongly stimulated in β-myrcene-supplemented medium, that is essential for β-myrcene catabolism. This island includes β-myrcene-induced genes whose products are putatively involved in 1) substrate sensing, 2) gene expression regulation, and 3) β-myrcene oxidation and bioconversion of β-myrcene derivatives into central metabolism intermediates. In general, this locus does not show high homology with sequences available in databases and seems to have evolved through the assembly of several functional blocks acquired from different bacteria, probably, at different evolutionary stages.

Microbial monoterpene transformations-a review

Isoprene and monoterpenes constitute a significant fraction of new plant biomass. Emission rates into the atmosphere alone are estimated to be over 500 Tg per year. These natural hydrocarbons are mineralized annually in similar quantities. In the atmosphere, abiotic photochemical processes cause lifetimes of minutes to hours. Microorganisms encounter isoprene, monoterpenes, and other volatiles of plant origin while living in and on plants, in the soil and in aquatic habitats. Below toxic concentrations, the compounds can serve as carbon and energy source for aerobic and anaerobic microorganisms. Besides these catabolic reactions, transformations may occur as part of detoxification processes. Initial transformations of monoterpenes involve the introduction of functional groups, oxidation reactions, and molecular rearrangements catalyzed by various enzymes. Pseudomonas and Rhodococcus strains and members of the genera Castellaniella and Thauera have become model organisms for the elucidation of biochemical pathways. We review here the enzymes and their genes together with microorganisms known for a monoterpene metabolism, with a strong focus on microorganisms that are taxonomically validly described and currently available from culture collections. Metagenomes of microbiomes with a monoterpene-rich diet confirmed the ecological relevance of monoterpene metabolism and raised concerns on the quality of our insights based on the limited biochemical knowledge.

The oxygen-independent metabolism of cyclic monoterpenes in Castellaniella defragrans 65Phen

Background

The facultatively anaerobic betaproteobacterium Castellaniella defragrans 65Phen utilizes acyclic, monocyclic and bicyclic monoterpenes as sole carbon source under oxic as well as anoxic conditions. A biotransformation pathway of the acyclic β-myrcene required linalool dehydratase-isomerase as initial enzyme acting on the hydrocarbon. An in-frame deletion mutant did not use myrcene, but was able to grow on monocyclic monoterpenes. The genome sequence and a comparative proteome analysis together with a random transposon mutagenesis were conducted to identify genes involved in the monocyclic monoterpene metabolism. Metabolites accumulating in cultures of transposon and in-frame deletion mutants disclosed the degradation pathway.

Results

Castellaniella defragrans 65Phen oxidizes the monocyclic monoterpene limonene at the primary methyl group forming perillyl alcohol. The genome of 3.95 Mb contained a 70 kb genome island coding for over 50 proteins involved in the monoterpene metabolism. This island showed higher homology to genes of another monoterpene-mineralizing betaproteobacterium, Thauera terpenica 58Eu^T, than to genomes of the family Alcaligenaceae, which harbors the genus Castellaniella. A collection of 72 transposon mutants unable to grow on limonene contained 17 inactivated genes, with 46 mutants located in the two genes ctmAB (cyclic terpene metabolism). CtmA and ctmB were annotated as FAD-dependent oxidoreductases and clustered together with ctmE, a 2Fe-2S ferredoxin gene, and ctmF, coding for a NADH:ferredoxin oxidoreductase. Transposon mutants of ctmA, B or E did not grow aerobically or anaerobically on limonene, but on perillyl alcohol. The next steps in the pathway are catalyzed by the geraniol dehydrogenase GeoA and the geranial dehydrogenase GeoB, yielding perillic acid. Two transposon mutants had inactivated genes of the monoterpene ring cleavage (mrc) pathway. 2-Methylcitrate synthase and 2-methylcitrate dehydratase were also essential for the monoterpene metabolism but not for growth on acetate.

Conclusions

The genome of Castellaniella defragrans 65Phen is related to other genomes of Alcaligenaceae, but contains a genomic island with genes of the monoterpene metabolism. Castellaniella defragrans 65Phen degrades limonene via a limonene dehydrogenase and the oxidation of perillyl alcohol. The initial oxidation at the primary methyl group is independent of molecular oxygen.

An oxygenase-independent cholesterol catabolic pathway operates under oxic conditions

Cholesterol is one of the most ubiquitous compounds in nature. The 9,10-seco-pathway for the aerobic degradation of cholesterol was established thirty years ago. This pathway is characterized by the extensive use of oxygen and oxygenases for substrate activation and ring fission. The classical pathway was the only catabolic pathway adopted by all studies on cholesterol-degrading bacteria. Sterolibacterium denitrificans can degrade cholesterol regardless of the presence of oxygen. Here, we aerobically grew the model organism with ¹³C-labeled cholesterol, and substrate consumption and intermediate production were monitored over time. Based on the detected ¹³C-labeled intermediates, this study proposes an alternative cholesterol catabolic pathway. This alternative pathway differs from the classical 9,10-seco-pathway in numerous important aspects. First, substrate activation proceeds through anaerobic C-25 hydroxylation and subsequent isomerization to form 26-hydroxycholest-4-en-3-one. Second, after the side chain degradation, the resulting androgen intermediate is activated by adding water to the C-1/C-2 double bond. Third, the cleavage of the core ring structure starts at the A-ring via a hydrolytic mechanism. The ¹⁸O-incorporation experiments confirmed that water is the sole oxygen donor in this catabolic pathway.

Physiology of deletion mutants in the anaerobic β-myrcene degradation pathway in Castellaniella defragrans

BACKGROUND

Monoterpenes present a large and versatile group of unsaturated hydrocarbons of plant origin with widespread use in the fragrance as well as food industry. The anaerobic β-myrcene degradation pathway in Castellaniella defragrans strain 65Phen differs from well known aerobic, monooxygenase-containing pathways. The initial enzyme linalool dehydratase-isomerase ldi/LDI catalyzes the hydration of β-myrcene to (S)-(+)-linalool and its isomerization to geraniol. A high-affinity geraniol dehydrogenase geoA/GeDH and a geranial dehydrogenase geoB/GaDH contribute to the formation of geranic acid.A genetic system was for the first time applied for the betaproteobacterium to prove in vivo the relevance of the linalool dehydratase-isomerase and the geraniol dehydrogenase. In-frame deletion cassettes were introduced by conjugation and two homologous recombination events.

RESULTS

Polar effects were absent in the in-frame deletion mutants C. defragrans Δldi and C. defragrans ΔgeoA. The physiological characterization of the strains demonstrated a requirement of the linalool dehydratase-isomerase for growth on acyclic monoterpenes, but not on cyclic monoterpenes. The deletion of geoA resulted in a phenotype with hampered growth rate on monoterpenes as sole carbon and energy source as well as reduced biomass yields. Enzyme assays revealed the presence of a second geraniol dehydrogenase. The deletion mutants were in trans complemented with the broad-host range expression vector pBBR1MCS-4ldi and pBBR1MCS-2geoA, restoring in both cases the wild type phenotype.

CONCLUSIONS

In-frame deletion mutants of genes in the anaerobic β-myrcene degradation revealed novel insights in the in vivo function. The deletion of a high-affinity geraniol dehydrogenase hampered, but did not preclude growth on monoterpenes. A second geraniol dehydrogenase activity was present that contributes to the β-myrcene degradation pathway. Growth on cyclic monoterpenes independent of the initial enzyme LDI suggests the presence of a second enzyme system activating unsaturated hydrocarbons.

Geraniol and geranial dehydrogenases induced in anaerobic monoterpene degradation by Castellaniella defragrans

Castellaniella defragrans is a Betaproteobacterium capable of coupling the oxidation of monoterpenes with denitrification. Geraniol dehydrogenase (GeDH) activity was induced during growth with limonene in comparison to growth with acetate. The N-terminal sequence of the purified enzyme directed the cloning of the corresponding open reading frame (ORF), the first bacterial gene for a GeDH (geoA, for geraniol oxidation pathway). The C. defragrans geraniol dehydrogenase is a homodimeric enzyme that affiliates with the zinc-containing benzyl alcohol dehydrogenases in the superfamily of medium-chain-length dehydrogenases/reductases (MDR). The purified enzyme most efficiently catalyzes the oxidation of perillyl alcohol (k(cat)/K(m) = 2.02 × 10(6) M(-1) s(-1)), followed by geraniol (k(cat)/K(m) = 1.57 × 10(6) M(-1) s(-1)). Apparent K(m) values of <10 μM are consistent with an in vivo toxicity of geraniol above 5 μM. In the genetic vicinity of geoA is a putative aldehyde dehydrogenase that was named geoB and identified as a highly abundant protein during growth with phellandrene. Extracts of Escherichia coli expressing geoB demonstrated in vitro a geranial dehydrogenase (GaDH) activity. GaDH activity was independent of coenzyme A. The irreversible formation of geranic acid allows for a metabolic flux from β-myrcene via linalool, geraniol, and geranial to geranic acid.

Linalool dehydratase-isomerase, a bifunctional enzyme in the anaerobic degradation of monoterpenes

Castellaniella (ex Alcaligenes) defragrans strain 65Phen mineralizes monoterpenes in the absence of oxygen. Soluble cell extracts anaerobically catalyzed the isomerization of geraniol to linalool and the dehydration of linalool to myrcene. The linalool dehydratase was present in cells grown on monoterpenes, but not if grown on acetate. We purified the novel enzyme ∼1800-fold to complete homogeneity. The native enzyme had a molecular mass of 160 kDa. Denaturing gel electrophoresis revealed one single protein band with a molecular mass of 40 kDa, which indicated a homotetramer as native conformation. The aerobically purified enzyme was anaerobically activated in the presence of 2 mm DTT. The linalool dehydratase catalyzed in vitro two reactions in both directions depending on the thermodynamic driving forces: a water secession from the tertiary alcohol linalool to the corresponding acyclic monoterpene myrcene and an isomerization of the primary allylalcohol geraniol in its stereoisomer linalool. The specific activities (V(max)) were 140 nanokatals mg(-1) for the linalool dehydratase and 410 nanokatals mg(-1) for the geraniol isomerase, with apparent K(m) values of 750 μm and 500 μm, respectively. The corresponding open reading frame was identified and revealed a precursor protein with a signal peptide for a periplasmatic location. The amino acid sequence did not affiliate with any described enzymes. We suggest naming the enzyme linalool dehydratase-isomerase according to its bifunctionality and placing it as a member of a new protein family within the hydrolyases (EC 4.2.1.X).

Biochemical characterization of AtuD from Pseudomonas aeruginosa, the first member of a new subgroup of acyl-CoA dehydrogenases with specificity for citronellyl-CoA

The atuRABCDEFGH gene cluster is essential for acyclic terpene utilization (Atu) in Pseudomonas aeruginosa. The biochemical functions of most Atu proteins have not been experimentally verified; exceptions are AtuC/AtuF, which constitute the two subunits of geranyl-CoA carboxylase, the key enzyme of the Atu pathway. In this study we investigated the biochemical function of AtuD and of the PA1535 gene product, a protein related to AtuD in amino acid sequence. 2D gel electrophoresis showed that AtuD and the PA1535 protein were specifically expressed in cells grown on acyclic terpenes but were absent in isovalerate- or succinate-grown cells. Mutant analysis indicated that AtuD but not the product of PA1535 is essential for acyclic terpene utilization. AtuD and PA1535 gene product were expressed in recombinant Escherichia coli and purified to homogeneity. Purified AtuD showed citronellyl-CoA dehydrogenase activity (V(max) 850 mU mg(-1)) and high affinity to citronellyl-CoA (K(m) 1.6 microM). AtuD was inactive with octanoyl-CoA, 5-methylhex-4-enoyl-CoA or isovaleryl-CoA. Purified PA1535 gene product revealed high citronellyl-CoA dehydrogenase activity (V(max) 2450 mU mg(-1)) but had significantly lower affinity than AtuD to citronellyl-CoA (K(m) 18 microM). Purified PA1535 protein additionally utilized octanoyl-CoA as substrate (V(max), 610 mU mg(-1); K(m) 130 microM). To our knowledge AtuD is the first acyl-CoA dehydrogenase with a documented substrate specificity for terpenoid molecule structure and is essential for a functional Atu pathway. Potential other terpenoid-CoA dehydrogenases were found in the genomes of Pseudomonas citronellolis, Marinobacter aquaeolei and Hahella chejuensis but were absent in non-acyclic terpene-utilizing bacteria.

Initial Steps in the Anoxic Metabolism of Cholesterol by the Denitrifying Sterolibacterium denitrificans

The anoxic metabolism of the ubiquitous triterpene cholesterol is challenging because of its complex chemical structure, low solubility in water, low number of active functional groups, and the presence of four alicyclic rings and two quaternary carbon atoms. Consequently, the aerobic metabolism depends on oxygenase catalyzed reactions requiring molecular oxygen as co-substrate. Sterolibacterium denitrificans is shown to metabolize cholesterol anoxically via the oxidation of ring A, followed by an oxygen-independent hydroxylation of the terminal C-25 of the side chain. The anaerobic hydroxylation of a tertiary carbon using water as oxygen donor is unprecedented and may be catalyzed by a novel molybdenum containing enzyme.

Biodegradation of alpha-pinene in model biofilms in biofilters

Treatment of air pollutants in a biofilter requires that the compound be effectively transported from the gas phase to the organisms that reside in a biofilm that forms upon a packing material. Models of biofiltration generally treat the biofilm like water by using a Henry's law constant to predict mass transfer rates into the biofilm where degradation occurs and, hence, predict low rates for hydrophobic compounds. However, some compounds that are virtually insoluble in water are also treated unusually well. The objective of this study was to develop a fundamental understanding of the apparent enhanced degradation of hydrophobic pollutants in biofilms. Specifically, the goals of this study were to experimentally determine transport and reaction rates of hydrophobic pollutants in artificial biofilms. We studied the transport and reaction rates of alpha-pinene (as a model hydrophobic pollutant) in a headspace in contact with a well-defined biofilm made up of biomass immobilized in low melting point agarose and found that reaction rates were similar in order of magnitude to biofilter rates. The transport rates through these films once deactivated were found to be the same as through agar (diffusion coefficient between 2.6 and 3.4 x 10(-6) cm2/s). The degradation rates through model biofilms ranged from 2 to 4 x 10(-7) (g/(cm2 min)). A new explanation of high degradation rates was put forth whereby a biologically mediated transformation is taking place in which alpha-pinene is oxidized into a more soluble, less volatile compound that can then penetrate deeper into the biofilm. The formation of this more soluble byproduct was confirmed with batch kinetics experiments using filtered samples, and its proposed identity is cis-2,8-p-menthadien-1-ol, a menthadienol, a novel metabolite of alpha-pinene degradation. A simple conceptual model based on these results is also presented.

The gnyRDBHAL cluster is involved in acyclic isoprenoid degradation in Pseudomonas aeruginosa

Pseudomonas aeruginosa PAO1 mutants affected in the ability to degrade acyclic isoprenoids were isolated with transposon mutagenesis. The gny cluster (for geranoyl), which encodes the enzymes involved in the lower pathway of acyclic isoprenoid degradation, was identified. The gny cluster is constituted by five probable structural genes, gnyDBHAL, and a possible regulatory gene, gnyR. Mutations in the gnyD, gnyB, gnyA, or gnyL gene caused inability to assimilate acyclic isoprenoids of the citronellol family of compounds. Transcriptional analysis showed that expression of the gnyB gene was induced by citronellol and repressed by glucose, whereas expression of the gnyR gene had the opposite behavior. Western blot analysis of citronellol-grown cultures showed induction of biotinylated proteins of 70 and 73 kDa, which probably correspond to 3-methylcrotonoyl-coenzyme A (CoA) carboxylase and geranoyl-CoA carboxylase (GCCase) alpha subunits, respectively. The 73-kDa biotinylated protein, identified as the alpha-GCCase subunit, is encoded by gnyA. Intermediary metabolites of the isoprenoid pathway, citronellic and geranic acids, were shown to accumulate in gnyB and gnyA mutants. Our data suggest that the protein products encoded in the gny cluster are the beta and alpha subunits of geranoyl-CoA carboxylase (GnyB and GnyA), the citronelloyl-CoA dehydrogenase (GnyD), the gamma-carboxygeranoyl-CoA hydratase (GnyH), and the 3-hydroxy-gamma-carboxygeranoyl-CoA lyase (GnyL). We conclude that the gnyRDBHAL cluster is involved in isoprenoid catabolism.