Molecular biology of Bacillus subtilis cytochromes

Promoted evolution of a shortened variant of heme A synthase in the membrane of Bacillus subtilis

Bacillus subtilis heme A synthase is a membrane protein with 8 transmembrane segments. By using a two-step mutagenesis approach we have generated and selected a fully functional enzyme protein variant with a seven residue internal deletion. The biochemical properties of the shortened variant are similar to those of the normal enzyme. This could indicate that residue H209 in the mutant protein substitutes for the missing H216 as an axial ligand to the heme iron. Our results provide insight in routes of membrane protein evolution and the structure of heme A synthases.

The response regulator ResD modulates virulence gene expression in response to carbohydrates in Listeria monocytogenes

Listeria monocytogenes is a versatile bacterial pathogen that is able to accommodate to diverse environmental and host conditions. Presently, we have identified a L. monocytogenes two-component response regulator, ResD that is required for the repression of virulence gene expression known to occur in the presence of easily fermentable carbohydrates not found inside host organisms. Structurally and functionally, ResD resembles the respiration regulator ResD in Bacillus subtilis as deletion of the L. monocytogenes resD reduces respiration and expression of cydA, encoding a subunit of cytochrome bd. The resD mutation also reduces expression of mptA encoding the EIIABman component of a mannose/glucose-specific PTS system, indicating that ResD controls sugar uptake. This notion was supported by the poor growth of resD mutant cells that was alleviated by excess of selected carbohydrates. Despite the growth deficient phenotype of the mutant in vitro the mutation did not affect intracellular multiplication in epithelial or macrophage cell lines. When examining virulence gene expression we observed traditional induction by charcoal in both mutant and wild-type cells whereas the repression observed in wild-type cells by fermentable carbohydrates did not occur in resD mutant cells. Thus, ResD is a central regulator of L. monocytogenes when present in the external environment.

Bacillus subtilis CtaA and CtaB function in haem A biosynthesis

Haem A, a prosthetic group of many respiratory oxidases, is probably synthesized from haem B (protohaem IX) in a pathway in which haem O is an intermediate. Possible roles of the Bacillus subtilis ctaA and ctaB gene products in haem O and haem A synthesis were studied. Escherichia coli does not contain haem A. The ctaA gene on plasmids in E. coli resulted in haem A accumulation in membranes. The presence of ctaB together with ctaA increased the amount of haem A found in E. coli. Haem O was not detected in wild-type B. subtilis strains. A previously isolated B. subtilis ctaA deletion mutant was found to contain haem B and haem O, but not haem A. B. subtilis ctaB deletion mutants were constructed and found to lack both haem A and haem O. The results with E. coli and B. subtilis strongly suggest that the B. subtilis CtaA protein functions in haem A synthesis. It is tentatively suggested that if functions in the oxygenation/oxidation of the methyl side group of carbon 8 of haem O. B. subtilis CtaB, which is homologous to Saccharomyces cerevisiae COX10 and E. coli CyoE, also has a role in haem A synthesis and seems to be required for both cytochrome a and cytochrome o synthesis.

Cloning sequencing and expression of the gene for cytochrome P450meg, the steroid-15 beta-monooxygenase from Bacillus megaterium ATCC 13368

A 4.3 kb EcoRI fragment carrying the gene for cytochrome P450meg, the steroid-15 beta-monooxygenase from Bacillus megaterium ATCC 13368, was cloned and completely sequenced. The gene codes for a protein of 410 amino acids and was expressed in Escherichia coli and B. subtilis. Protein extracts from the recombinant E. coli strains were able to hydroxylate corticosteroids in the 15 beta position when supplemented with an extract from a P450- mutant of B. megaterium ATCC 13368 as a source of megaredoxin and megaredoxin reductase. In contrast, 15 beta-hydroxylation was obtained in vitro and in vivo without the addition of external electron transfer proteins, when cytochrome P450meg was produced in B. subtilis 168. Protein extracts from nonrecombinant B. subtilis 168 could also support the in vitro hydroxylation by cytochrome P450meg produced in E. coli.