NUCLEOTIDE-SEQUENCE AND FUNCTIONAL-ANALYSIS OF CBBR, A POSITIVE REGULATOR OF THE CALVIN CYCLE OPERONS OF RHODOBACTER-SPHAEROIDES

Structural genes encoding Calvin cycle enzymes in Rhodobacter sphaeroi des are duplicated and organized within two physically distinct transc riptional units, the form I and form II cbb operons. Nucleotide sequen ce determination of the region upstream of the form I operon revealed a divergently transcribed open reading frame, cbbR, that showed signif icant similarity to the LysR family of transcriptional regulatory prot eins. Mutants containing an insertionally inactivated cbbR gene were i mpaired in photoheterotrophic growth and completely unable to grow pho tolithoautotrophically with CO, as the sole carbon source. In the cbbR strain, expression of genes within the form I operon was completely a bolished and that of the form II operon was reduced to about 30% of th e wild-type level. The cloned cbbR gene complemented the mutant for wi ld-type growth characteristics, and normal levels of ribulose 1,5-bisp hosphate carboxylase/oxygenase (RubisCO) were observed. However, rocke t immunoelectrophoresis revealed that the wild-type level of RubisCO w as due to overexpression of the form II enzyme, whereas expression of the form I RubisCO was 10% of that of the wild-type strain. The cbbR i nsertional inactivation did not appear to affect aerobic expression of either CO2 fixation operon, but preliminary evidence suggests that th e constitutive expression of the form II operon observed in the cbbR s train may be subject to repression during aerobic growth.