Structure and organization of a 25 kbp region of the genome of the photosynthetic green sulfur bacterium Chlorobium vibrioforme containing Mg-chelatase encoding genes

A region comprising approximately 25 kbp of the genome of the strictly anae robic and obligate photosynthetic green sulfur bacterium Chlorobium vibriof orme has been mapped, subcloned and partly sequenced. Approximately 15 kbp have been sequenced in it's entirety and three genes with significant homol ogy and feature similarity to the bchI, -D and -H genes and the cilI, -D an d -H genes of Rhodobacter and Synechocystis strain PCC6803, respectively, w hich encode magnesium chelatase subunits, have been identified. Magnesium c helatase catalyzes the insertion of Mg2+ into protoporphyrin IX, and is the first enzyme unique to the (bacterio)chlorophyll specific branch of the po rphyrin biosynthetic pathway. The organization of the three Mg-chelatase en coding genes is unique to Chlorobium and suggests that the magnesium chelat ase of C. vibrioforme is encoded by a single operon. The analyzed 25 kbp re gion contains five additional open reading frames, two of which display sig nificant homology and feature similarity to genes encoding lipoamide dehydr ogenase and genes with function in purine synthesis, and another three disp lay significant homology to open reading frames with unknown function in di stantly related bacteria. Putative E. coli sigma(70)-like promoter sequence s, ribosome binding sequences and rho-independent transcriptional stop sign als within the sequenced 15 kbp region are related to the identified genes and orfs. Southern analysis, restriction mapping and partial sequencing of the remaining ca. 10 kbp of the analyzed 25 kbp region have shown that this part includes the hemA, -C, -D and -B genes (MOBERG and AVISSAR 1994), whi ch encode enzymes with function in the early part of the biosynthetic pathw ay of porphyrins.