RHODOBACTER-CAPSULATUS GENES ENCODING FORM-I RIBULOSE-1,5-BISPHOSPHATE CARBOXYLASE OXYGENASE (CBBLS) AND NEIGHBORING GENES WERE ACQUIRED BYA HORIZONTAL GENE-TRANSFER/

Analysis of the nucleotide sequence of the form I ribulose-1,5-bisphos phate carboxylase/oxygenase (RubisCO) genes (cbbL and cbbS) of the non -sulfur purple bacterium Rhodobacter capsulatus indicated that the ded uced amino acid sequence of the large subunit was not closely homologo us to the large subunit from related organisms. Indeed, phylogenetic a nalysis suggested that the large subunit protein (CbbL) more closely r esembled the enzyme from alpha/beta/gamma purple bacteria and cyanobac teria and is within a 'green-like' radiation of the RubisCO phylogenet ic tree, well separated from CbbL of the related organism Rhodobacter sphaeroides. A cbbQ gene was discovered downstream of cbbS in Rh. caps ulatus, a gene arrangement which also appears to be limited to certain organisms containing a 'green-like' RubisCO. Upstream, and divergentl y transcribed from cbbLSQ, is a gene (cbbR(I)) that encodes a LysR-typ e transcriptional activator. Phylogenetic analysis of the deduced amin o acid sequence of CbbR(I) also suggests that this protein is quite di stinct from the Rh. sphaeroides CbbR protein, and is even distinct fro m the previously described CbbR(II) protein, the gene of which is upst ream and divergently transcribed from the cbb(II) operon of Rh. capsul atus. Interestingly, Rh. capsulatus CbbR(I) is more closely related to CbbR from bacteria whose RubisCO falls within the 'green-like' radiat ion of the CbbL tree. These studies suggest that the cbbR(I)-cbbL-cbbS -cbbQ genes were acquired by Rh. capsulatus via horizontal gene transf er from a bacterial species containing a 'green-like' RubisCO.