Control of hemA expression in Rhodobacter sphaeroides 2.4.1: Effect of a transposon insertion in the hbdA gene

The common precursor to all tetrapyrroles is 5-aminolevulinic acid (ALA), a nd in Rhodobacter sphaeroides its formation occurs via the Shemin pathway. ALA synthase activity is encoded by two differentially regulated genes in R . sphaeroides 2.4.1: hemA and hemT. In our investigations of hemA regulatio n, we applied transposon mutagenesis under aerobic conditions, followed by a selection that identified transposon insertion mutants in which hemA expr ession is elevated. One of these mutants has been characterized previously (J. Zeilstra-Ryalls and S. Kaplan, J. Bacteriol. 178:985-993, 1996), and he re we describe our analysis of a second mutant strain. The transposon inser ted into the coding sequences of hbdA, coding for S-(+)-beta -hydroxybutryl -coenzyme A dehydrogenase and catalyzing an NAD-dependent reaction. We prov ide evidence that the hbdA gene product participates in polyhydroxybutyrate (PHB) metabolism and, based on our findings, we discuss possibilities as t o how defective PHB metabolism might alter the level of hemA expression.