Isolation and characterization of trans-acting mutations involved in oxygen regulation of puc operon transcription in Rhodobacter sphaeroides.

Transcriptional expression of the puc operon in Rhodobacter sphaeroides 2.4.1 is dependent on the partial pressure of oxygen. By using transcriptional fusions in trans of a promoterless fragment derived from the aminoglycoside-3'-phosphotransferase gene of Tn903 to puc operon-specific DNA containing a 629-bp 5' cis-acting regulatory region involved in the expression of puc-specific mRNA, we selected Kmr colonies under aerobic conditions. Two broad classes of mutations, trans and cis, which are involved in O2 control of puc operon transcription, fall into several distinct phenotypic classes. The cis-acting regulatory mutations are characterized in detail elsewhere (J.K. Lee and S. Kaplan, J. Bacteriol. 174:1146-1157, 1992). Two trans-acting regulatory mutants, CL1a and T1a, which are B800-850- Car- and apparently B875-, respectively, were shown to derepress puc operon transcription in the presence of oxygen. The mutation giving rise to CL1a has been shown to act at the puc operon-specific cis-acting upstream regulatory region (-629 to -92). On the other hand, the mutation giving rise to T1a, identifying a second trans-acting regulatory factor(s), appears to act at both the upstream (-629 to -92) and the downstream (-92 to -1) regulatory regions of the puc operon as well as at the level(s) of bacteriochlorophyll and carotenoid biosyntheses, as revealed by the presence of the B800-850 complex under chemoheterotrophic growth conditions. Both the B800-850- Car- phenotype and the trans-acting effect on puc operon expression in mutant CL1a were complemented with a 2.2-kb DNA fragment located within the carotenoid gene cluster. Mutant T1a was complemented with a 7.0-kb EcoRI restriction fragment containing the puhA gene and its flanking DNA (6.3 kb) to restore expression of the B875 complex and to suppress the trans-acting effect resulting in the loss of 02 control. Under chemoheterotrophic conditions, mutant T1a was highly unstable, segregating into a PS- mutant designated T4.