Quaternary structure of delta-aminolevulinic acid synthase from Rhodopseudomonas spheroides

Biochemistry, regulation and genomics of haem biosynthesis in prokaryotes

Haems are involved in many cellular processes in prokaryotes and eukaryotes. The biosynthetic pathway leading to haem formation is, with few exceptions, well-conserved, and is controlled in accordance with cellular function. Here, we review the biosynthesis of haem and its regulation in prokaryotes. In addition, we focus on a modification of haem for cytochrome c biogenesis, a complex process that entails both transport between cellular compartments and a specific thioether linkage between the haem moiety and the apoprotein. Finally, a whole genome analysis from 63 prokaryotes indicates intriguing exceptions to the universality of the haem biosynthetic pathway and helps define new frontiers for future study.

Expression of the Rhodobacter sphaeroides hemA and hemT genes, encoding two 5-aminolevulinic acid synthase isozymes

The nucleotide sequences of the Rhodobacter sphaeroides hemA and hemT genes, encoding 5-aminolevulinic acid (ALA) synthase isozymes, were determined. ALA synthase catalyzes the condensation of glycine and succinyl coenzyme A, the first and rate-limiting step in tetrapyrrole biosynthesis. The hemA and hemT structural gene sequences were 65% identical to each other, and the deduced HemA and HemT polypeptide sequences were 53% identical, with an additional 16% of aligned amino acids being similar. HemA and HemT were homologous to all characterized ALA synthases, including two human ALA synthase isozymes. In addition, they were evolutionarily related to 7-keto-8-aminopelargonic acid synthetase (BioF) and 2-amino-3-ketobutyrate coenzyme A ligase (Kbl), enzymes which catalyze similar reactions. Two hemA transcripts were identified, both expressed under photosynthetic conditions at levels approximately three times higher than those found under aerobic conditions. A single transcriptional start point was identified for both transcripts, and a consensus sequence at this location indicated that an Fnr-like protein may be involved in the transcriptional regulation of hemA. Transcription of hemT was not detected in wild-type cells under the physiological growth conditions tested. In a mutant strain in which the hemA gene had been inactivated, however, hemT was expressed. In this mutant, hemT transcripts were characterized by Northern (RNA) hybridization, primer extension, and ribonuclease protection techniques. A small open reading frame of unknown function was identified upstream of, and transcribed in the same direction as, hemA.

Cloning and characterization of the 5-aminolevulinate synthase gene(s) from Rhodobacter sphaeroides

The 5-aminolevulinate synthase gene (hemA) from Rhizobium meliloti was used to probe a genomic lambda bank derived from Rhodobacter sphaeroides DNA. Two phage clones were found to bear homology to the Rhizobium probe. Southern hybridization analysis of the two lambda phage clones, which we designated lambda Hem 10 and lambda Hem 12, showed that the homology to the Rhizobium hemA gene was localized to a 3.1-kb SalI fragment derived from lambda Hem 10 and a 7.0-kb SalI fragment derived from lambda Hem 12. Each of the SalI fragments was subsequently cloned into the multiple cloning site of pUC19 in both orientations relative to the lac promoter. Restriction analysis confirmed that each SalI fragment was unique. It was also shown from Southern hybridization analysis that the regions of homology within each of the R. sphaeroides restriction fragments and the Rhizobium probe were different. Further, we have tentatively concluded that each R. sphaeroides hemA gene shows a relatively low degree of homology to the other. Data obtained from in vitro transcription-translation studies in a homologous R. sphaeroides cell-free system, and complementation of hemA mutations of both Escherichia coli and R. sphaeroides by either of the putative hemA clones suggested the presence of a gene encoding 5-aminolevulinate synthase on each DNA sequence. The fact that 5-aminolevulinate synthase activity could be demonstrated in mutant strains complemented in trans with either cloned DNA fragment further supported this conclusion.

Inhibition of bacteriochlorophyll synthesis in Rhodobacter sphaeroides subsp. denitrificans grown in light under denitrifying conditions

The inclusion of nitrate or nitrite in cultures of Rhodobacter spaeroides subsp. denitrificans grown heterotrophically in light depressed the formation of bacteriochlorophyll a. The pigment biosynthesis was inhibited at the stage of the reduction of chlorophyllide (chlorin) to bacteriochlorophyllide (tetrahydroporphyrin) since 3-hydroxyethylchlorophyllide a accumulated in the culture medium. The addition of exogenous 5-aminolevulinic acid to these cultures resulted in a complete restoration of bacteriochlorophyll synthesis accompanied by the accumulation of 3-vinylbacteriopheophorbide. This indicates that under these conditions bacteriochlorophyll was formed via an alternative route, in which the reduction of chlorins to tetrahydroporphyrins precedes modifications of the C-3 side chain. The multiple forms of 5-aminolevulinic acid synthase were purified from cells grown with and without nitrate. Antibodies against these proteins were raised in rabbits and used in enzyme-linked immunosorbent assays for various forms of 5-aminolevulinic acid synthase. In denitrifying cells, the amount and activity of fraction I of the enzyme was reduced by approximately 40 and 30%, respectively. Partly active enzymes from both types of cells were activated by cystine trisulfide.