Resolving the roles of the Rhodobacter sphaeroides HemA and HemT 5-aminolevulinic acid synthases

Strains of the phototrophic alpha-proteobacterium Rhodobacter sphaeroides vary in the number of enzymes catalyzing the formation of 5-aminolevulinic acid (ALA synthases) that are encoded in their genomes. All have hemA, but not all have hemT. This study compared transcription of these genes, and also properties of their products among three wild-type strains; 2.4.3 has hemA alone, 2.4.1 and 2.4.9 have both hemA and hemT. Using lacZ reporter plasmids all hemA genes were found to be upregulated under anaerobic conditions, but induction amplitudes differ. hemT is transcriptionally silent in 2.4.1 but actively transcribed in 2.4.9, and strongly upregulated under anaerobic-dark growth conditions when cells are respiring dimethyl sulfoxide, vs. aerobic-dark or phototrophic (anaerobic-light) conditions. Two extracytoplasmic function (ECF)-type sigma factors present in 2.4.9, but absent from 2.4.1 are directly involved in hemT transcription. Kinetic properties of the ALA synthases of all three strains were similar, but HemT enzymes are far less sensitive to feedback inhibition by hemin than HemA enzymes, and HemT is less active under oxidizing conditions. A model is presented that compares and contrast events in strains 2.4.1 and 2.4.9.