Transcriptional regulation of delta-aminolevulinic acid dehydratase synthesis by oxygen in Bradyrhizobium japonicum and evidence for developmental control of the hemB gene

An increased demand for cytochromes is associated with symbiotic development and microaerobic metabolism in the bacterium Bradyrhizobium japonicum, and evidence suggests that hemB, rather than hemA, is the first essential bacterial heme synthesis gene in symbiosis with soybean. Steady-state levels of mRNA and protein encoded by hemB were strongly and rapidly induced by O-2 deprivation as determined by RNase protection and immunoblot analyses, but hemH message was not induced. Oxygen limitation resulted in a greater-than-10-fold increase in the rate of hemB mRNA synthesis as determined by transcriptional runoff experiments, whereas hemH transcription was unaffected by the O-2 status. Thus, hemB is a regulated gene in B. japonicum and is transcriptionally controlled by O-2. Unlike the expression in parent strain I110, hemB expression was not affected by O-2 in the fixJ strain 7360, and O-2-limited cultures of the mutant contained quantities of hemB mRNA and protein that were comparable to uninduced levels found in aerobic cells. In addition, spectroscopic analysis of cell extracts showed that increases in b- and c-type cytochromes and the disappearance of cytochrome aa, in response to microaerobic growth in wild-type cells were not observed in the fixJ mutant. FixJ is a key transcriptional regulator that mediates O-2-dependent differentiation in rhizobia, and therefore hemB expression is under developmental control. Furthermore, the data suggest a global control of cytochrome expression and heme biosynthesis in response to the cellular O-2 status.