Rhodobacter capsulatus genes encoding form I ribulose-1,5-bisphosphate carboxylase/oxygenase (cbbLS) and neighbouring genes were acquired by a horizontal gene transfer

Abstract

Summary: Analysis of the nucleotide sequence of the form I ribulose-1,5-bisphosphate carboxylase/oxygenase (RubisCO) genes (cbbL and cbbS) of the non-sulfur purple bacterium Rhodobacter capsulatus indicated that the deduced amino acid sequence of the large subunit was not closely homologous to the large subunit from related organisms. Indeed, phylogenetic analysis suggested that the large subunit protein (CbbL) more closely resembled the enzyme from αlβlγ purple bacteria and cyanobacteria and is within a ‘green-like’ radiation of the RubisCO phylogenetic tree, well separated from CbbL of the related organism Rhodobacter sphaeroides. A cbbQ gene was discovered downstream of cbbS in Rh. capsulatus, a gene arrangement which also appears to be limited to certain organisms containing a ‘green-like’ RubisCO. Upstream, and divergently transcribed from cbbLSQ, is a gene (cbbR _I) that encodes a LysR-type transcriptional activator. Phylogenetic analysis of the deduced amino acid sequence of CbbR_I also suggests that this protein is quite distinct from the Rh. sphaeroides CbbR protein, and is even distinct from the previously described CbbR_II protein, the gene of which is upstream and divergently transcribed from the cbb _II operon of Rh. capsulatus. Interestingly, Rh. capsulatus CbbR_I is more closely related to CbbR from bacteria whose RubisCO falls within the ‘greenlike’ radiation of the CbbL tree. These studies suggest that the cbbR _I-cbbL-cbbS-cbbQ genes were acquired by Rh. capsulatus via horizontal gene transfer from a bacterial species containing a ‘green-like’ RubisCO.