Cloning of Genes Involved in the Biosynthesis of Bacteriochlorophyll and Heme in Chlorobium Tepidum

Abstract

The biosynthesis of chlorophyll and bacteriochlorophyll follows a pathway which, up to protoporphyrin IX, is identical to the pathway of heme biosynthesis (Fig. 1). The formation of 5-aminolevulinate can be achieved either from glycine and succinyl-CoA, in a reaction catalysed by the enzyme 5-aminolevulinic acid synthase, or by a three step pathway from glutamate catalysed by three enzymes, of which the second is glutamyl-tRNA dehydrogenase (1). The succinyl-CoA pathway is used by nonplant eukaryotes and a group of bacteria that includes the purple photosynthetic bacteria Rhodobacter and Rhodospirillum as well as nonphotosynthetic relatives such as Agrobacterium and Rhizobium. The glutamate pathway is used by plants and green photosynthetic sulfur bacteria (Chlorobiaceae). In photosynthetic organisms Mg-chelatase, which catalyses the ATP-dependent insertion of Mg2+ into protoporphyrin IX, is the first enzyme unique to the biosynthesis of chlorophyll (Fig. 1). Therefore this enzyme is believed to play an important regulatory role in channelling intermediates of the pathway into either the heme or the chlorophyll specific branch in response to various physiological signals and requirements. Open image in new window Figure 1 The biosynthetic pathway for heme and bacteriochlorophyll (or chlorophyll). Only some of the intermediates and enzymes are shown. The genes encoding some of the enzymes are given in parentheses. (adapted from ref. 6)