Evidence that a linear megaplasmid encodes enzymes of aliphatic alkene and epoxide metabolism and coenzyme M (2-mercaptoethanesulfonate) biosynthesis in Xanthobacter strain Py2

The bacterial metabolism of propylene proceeds by epoxidation to epoxypropane followed by a sequence of three reactions resulting in epoxide ring opening and carboxylation to form acetoacetate. Coenzyme M (2-mercaptoethanesulfonic acid) (CoM) plays a central role in epoxide carboxylation by serving as the nucleophile for epoxide ring opening and the carrier of the C(3) unit that is ultimately carboxylated to acetoacetate, releasing CoM. In the present work, a 320-kb linear megaplasmid has been identified in the gram-negative bacterium Xanthobacter strain Py2, which contains the genes encoding the key enzymes of propylene oxidation and epoxide carboxylation. Repeated subculturing of Xanthobacter strain Py2 under nonselective conditions, i.e., with glucose or acetate as the carbon source in the absence of propylene, resulted in the loss of the propylene-positive phenotype. The propylene-negative phenotype correlated with the loss of the 320-kb linear megaplasmid, loss of induction and expression of alkene monooxgenase and epoxide carboxylation enzyme activities, and the loss of CoM biosynthetic capability. Sequence analysis of a hypothetical protein (XecG), encoded by a gene located downstream of the genes for the four enzymes of epoxide carboxylation, revealed a high degree of sequence identity with proteins of as-yet unassigned functions in the methanogenic archaea Methanobacterium thermoautotrophicum and Methanococcus jannaschii and in Bacillus subtilis. The M. jannaschii homolog of XecG, MJ0255, is located next to a gene, MJ0256, that has been shown to encode a key enzyme of CoM biosynthesis (M. Graupner, H. Xu, and R. H. White, J. Bacteriol. 182: 4862-4867, 2000). We propose that the propylene-positive phenotype of Xanthobacter strain Py2 is dependent on the selective maintenance of a linear megaplasmid containing the genes for the key enzymes of alkene oxidation, epoxide carboxylation, and CoM biosynthesis.