TRANSCRIPTIONAL CONTROL OF SEVERAL AEROBICALLY INDUCED CYTOCHROME STRUCTURAL GENES IN RHODOBACTER-SPHAEROIDES

To decipher how the synthesis of energy-transducing enzymes responds t o environmental cues, the response of three Rhodobacter sphaeroides ae robic cytochrome gene promoters was analysed under different condition s. Two of these promoters are upstream of structural genes (ctaD and c oxII) for individual subunits of the cytochrome aa(3) respiratory comp lex. The third promoter is that for the cycFG operon, which encodes tw o c-type cytochromes of unknown function, cytochrome c(554) and CycG. Primer extension analysis identified a single oxygen-responsive transc ription start site for each gene. Utilizing operon fusions to Escheric hia coli lacZ as a measure of promoter activity, transcription from th e ctaD, coxII and cycFG promoters was approximately twofold higher whe n cells were grown at high (30%) oxygen tensions than under low (2%) o xygen or anaerobic (photosynthetic) conditions. Analysis of promoter f unction using specific host mutations indicated that loss of the R. sp haeroides FNR homologue, FnrL, causes a small, but reproducible, incre ase in cycFG and coxII transcription when cells are grown at 2% oxygen . However, neither the Delta FnrL mutation nor alterations in sequence s related to a consensus target site for the E. coli FNR protein incre ased function of any of these three promoters to that seen under aerob ic conditions in wild-type cells. From this we conclude that FnrL is n ot solely responsible for reduced transcription of these three aerobic cytochrome genes under low oxygen or anaerobic conditions. When activ ity of these three promoters was monitored after cells were shifted fr om anaerobic (photosynthetic) conditions to a 30% oxygen atmosphere, i t took several cell doublings for LacZ levels to increase to those fou nd in steady-state 30% oxygen cultures. From these results, it appears that activity of these promoters is also regulated by a stable molecu le whose synthesis or function responds slowly to the presence of high oxygen tensions.