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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  Δ ldi  and  C. defragrans  Δ geoA . 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-4 ldi  and pBBR1MCS-2 geoA , 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.

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