UNUSUAL PATHWAYS AND ENVIRONMENTALLY CONT ROLLED GENES OF BACTERIAL HEME-BIOSYNTHESIS

The majority of bacteria, all investigated archaea and plants form the general precursor molecule of all tetrapyrroles 5-aminolevulinic acid by a unique transformation of transfer RNA bound glutamate. Only the alpha-group of the proteobacteria, mammals and yeast synthesize fi-ami nolevulinic acid via the well known condensation of succinyl-CoA and g lycine. The late steps in tetrapyrrole biosynthesis also contain alter native biosynthetic pathways for the formation and oxidative decarboxy lation of coproporphyrinogen III. Unusual enzymatic reactions includin g the utilization of two substrate molecules as cofactor by the porpho bilinogen deaminase and the formation of a spiro intermediate are invo lved in the formation of uroporphyrinogen III. The biosynthesis of hem es in bacteria is strictly regulated at the formation of 5-aminolevuli nic acid and the oxidative decarboxylation of coproporphyrinogen III. The involved heme biosynthetic genes are regulated by the environmenta l concentrations of oxygen, iron, nitrate, growth phase and intracellu lar levels of heme. The current knowledge on the various enzymatic rea ctions and gene regulatory mechanisms is reviewed.