Geranyl-coenzyme A (CoA)-carboxylase (GCase; AtuC/AtuF) and methylcrotonyl-CoA-carboxylase (MCase; LiuB/LiuD) are characteristic enzymes of the catabolic pathway of acyclic terpenes (citronellol and geraniol) and of saturated methyl-branched compounds, such as leucine or isovalerate, respectively. Proteins encoded by two gene clusters (atuABCDEFGH and liuRABCDE) of Pseudomonas aeruginosa PAO1 were essential for acyclic terpene utilization (Atu) and for leucine and isovalerate utilization (Liu), respectively, as revealed by phenotype analysis of 10 insertion mutants, two-dimensional gel electrophoresis, determination of GCase and MCase activities, and Western blot analysis of wild-type and mutant strains. Analysis of the genome sequences of other pseudomonads (P. putida KT2440 and P. fluorescens Pf-5) revealed candidate genes for Liu proteins for both species and candidate genes for Atu proteins in P. fluorescens. This result concurred with the finding that P. fluorescens, but not P. putida, could grow on acyclic terpenes (citronellol and citronellate), while both species were able to utilize leucine and isovalerate. A regulatory gene, atuR, was identified upstream of atuABCDEFGH and negatively regulated expression of the atu gene cluster.

Pseudomonas aeruginosa is able to grow on acyclic monoterpenes (citronellol, citronellate, geraniol and geranylate), and on other methyl-branched compounds such as leucine or isovalerate. The catabolic pathway of citronellol (Atu, acyclic terpene utilization) enters that of leucine/isovalerate (Liu, leucine and isovalerate utilization) at the level of methylcrotonyl-CoA. Key enzymes of the combined pathways are geranyl-CoA carboxylase (GCase) and methylcrotonyl-CoA carboxylase (MCase). In this study, isovalerate-grown cells specifically expressed MCase (apparent molecular mass of the biotin-containing subunit, 74 kDa) only, and the GCase biotin-containing subunit (71 kDa) was not detected. Citronellol- or citronellate-grown cells produced both carboxylases. Biotin-dependent proteins were purified from crude extracts by avidin-affinity chromatography, and assigned to the corresponding coding genes by trypsin fingerprint analysis. The two subunits of MCase corresponded to liuB/liuD (PA2014/PA2012) of the P. aeruginosa genome database, and atuC/atuF (PA2888/PA2891) encoded GCase subunits. This finding is contrary to that reported by others. The identified genes are part of two separate gene clusters [liuRABCDE (PA2011–PA2016) and atuABCDEFGH (PA2886–PA2893)] that are thought to encode most of the genes of the Atu and Liu pathways.

Mini-Tn5-induced mutants with defects in utilization of linear terpenes such as citronellol, geraniol, citronellate and/or geranylate were isolated from Pseudomonas aeruginosa. One mutant was unable to utilize geraniol but showed wild-type growth with the three other acyclic terpenes tested. The Tn5 insertion site of the mutant was determined by DNA sequencing. Comparison with the P. aeruginosa genome sequence revealed that PA3028, an ORF with high similarity on the amino acid level to molybdenum cofactor biosynthesis protein A2 (encoded by moeA2), was the target of mini-Tn5 in the mutant. Disruption of moeA2 in P. aeruginosa PAO1 wild-type by insertion mutagenesis resulted in the same geraniol-minus phenotype. The ability to utilize geraniol was restored to the mutant by conjugative transfer of PCR-cloned wild-type moeA2 on a broad-host-range plasmid. Growth of P. aeruginosa PAO1 on geraniol and geranial, but not on citronellol, citronellate or geranylate, was inhibited by the presence of 10 mM tungstate, a molybdenum-specific inhibitor. Inhibition by tungstate was prevented by addition of molybdate. The results indicate that at least one step in the oxidation of geraniol to geranic acid (geranial oxidation) is a molybdenum-dependent reaction in P. aeruginosa and is different from the molybdenum-independent oxidation of citronellol to citronellate.

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