IDENTIFICATION OF CBBB(C) AS AN ADDITIONAL DISTAL GENE OF THE CHROMOSOMAL CBB CO2 FIXATION OPERON FROM RALSTONIA-EUTROPHA

Ralstonia eutropha (formerly Alcaligenes eutrophus) strain H16 possess es two highly homologous cbb operons encoding most of the Calvin cycle enzymes. One copy of the operon is located on the chromosome, the oth er on the megaplasmid pHG1 of the organism. Sequence analysis of the r egion downstream of the presumptive 3'-terminal gene (cbbA(c)) of the chromosomal operon revealed the presence of an open reading frame comp rising 2,274 bp. Evidence is presented that this open reading frame is an additional distal gene (designated cbbB(c)) of the operon. In cont rast to the other genes of the operon, cbbB(c) is not duplicated in th e plasmid-borne operon. The deduced amino acid sequence of the cbbB(c) product (757 residues, molecular mass 83.17 kDa) showed the highest s imilarity to the large catalytic subunits of various bacterial formate dehydrogenases (FDH), suggesting that cbbB(c) might represent a struc tural FDH gene of R. eutropha. However, the properties of a cbbB(c) mu tant strain indicated that the potential gene product is not related t o known FDH of the organism. Transcriptional analysis in the homologou s host and heterologous expression in Escherichia coli demonstrated th at cbbB(c) is an active gene, which apparently has no essential functi on in the autotrophic metabolism of R. eutropha. The gene is a novel m ember of cbb operons in autotrophic bacteria.