CbbR, a LysR-type transcriptional activator, is required for expression of the autotrophic CO2 fixation enzymes of Xanthobacter flavus.

Xanthobacter flavus is able to grow autotrophically with the enzymes of the Calvin cycle for the fixation of CO2, which are specified by the cbbLSXFP gene cluster. Previously, the 5' end of an open reading frame (cbbR), displaying a high sequence similarity to the LysR family of regulatory proteins and transcribed divergently from cbbLSXFP, was identified (W. G. Meijer, A. C. Arnberg, H. G. Enequist, P. Terpstra, M. E. Lidstrom, and L. Dijkhuizen, Mol. Gen. Genet. 225:320-330, 1991). This paper reports the complete nucleotide sequence of cbbR and a functional characterization of the gene. The cbbR gene of X. flavus specifies a 333-amino-acid polypeptide, with a molecular weight of 35,971. Downstream from cbbR, the 3' end of an open reading frame displaying a high similarity to ORF60K from Pseudomonas putida and ORF261 from Bacillus subtilis was identified. ORF60K and ORF261 are located at the replication origin of the bacterial chromosome. Inactivation of cbbR, via the insertion of an antibiotic resistance gene, rendered X. flavus unable to grow autotrophically. This was caused not by an inability to oxidize autotrophic substrates (e.g., formate) but by a complete lack of expression of the cbb genes. The expression of the CbbR protein in Escherichia coli was achieved by placing cbbR behind a strong promoter and optimization of the translational signals of cbbR. CbbR binds specifically to two binding sites in the cbbR-cbbL intergenic region.