A cosmid gene bank of partially EcoRI-digested genomic DNA from Methylobacterium extorquens IBT no. 6 was screened for DNA fragments restoring polyhydroxyalkanoic-acid (PHA) accumulation in the PHA-negative mutant Alkaligenes eutrophus H16 PHB-4. The M. extorquens PHA-synthase structural gene phaCMex was mapped on a 23-kbp EcoRI fragment by complementation studies, by hybridization experiments with heterologous DNA probes from A. eutrophus H16 encoding for phaA, phaB and phaC and by nucleic acid sequence analysis. Evidence for the presence of genes for a beta-ketothiolase or an acetoacetyl-coenzyme A reductase on this fragment was not obtained. The nucleotide sequence of a 3.7-kbp region was obtained. It contained the entire 1.815-kbp phaCMex plus approximately each 900-bp upstream and downstream of phaCMex.PhaCMex encoded a protein of 605 amino acids with a relative molecular mass (M(r)) of 66742, which exhibited 38.1% amino acid identity with the A. eutrophus PHA synthase. Determination of the N-terminal amino acid sequence of an M(r) 65,000 protein, which was enriched concomitantly with the purification of PHA granules in sucrose gradients, revealed a sequence that was identical with the amino acid sequence deduced from the most probable translation start codon except for a valine, which was obviously removed post-translationally. Enzyme analysis, which was done with the native gene and a phaCMex'-'lacZ fusion gene, gave no evidence for expression of phaCMex in Escherichia coli.