Substitution of RNA-polymerase sigma-subunits and transcription regulation

Recent data on regulation of gene activity in bacteria by substitution of RNA polymerase sigma subunits are reviewed. The htpR gene which controls the switch-on of the Escherichia coli heat-shock protein synthesis codes for sigma 32 subunit. sigma 32-containing RNA polymerase transcribes the heat-shock genes in vitro from specific promoters of no use for RNA polymerase containing the major sigma 70 subunit. Several minor sigma subunits have been found in Bacillus subtilis vegetative cells, in addition to the major sigma 55 subunit, differing in the specificity of promoter recognition. Many B. subtilis genes are controlled by tandemly located promoters recognized by RNA polymerases carrying different sigma subunits. sigma 29 subunit is encoded by spoIIG gene and is probably involved in the regulation of sporulation. Specific sigma subunits for transcribing "middle" or "late" genes are encoded by a number of phages.     

The developmental fate of S. coelicolor hyphae depends upon a gene product homologous with the motility sigma factor of B. subtilis

In the mycelial prokaryote S. coelicolor, whiG is a gene dispensable for growth but needed for the earliest stages of spore formation in aerial hyphae. Nucleotide sequencing indicates that whiG encodes an RNA polymerase sigma factor highly similar to the motility sigma factor (sigma 28) of B. subtilis. High copy number of an intact whiG gene caused sporulation in vegetative hyphae that are usually fated to lyse without sporulating. However, the introduction of many copies of a sigma 28-dependent promoter from B. subtilis into S. coelicolor reduced sporulation, suggesting partial sequestration of the whiG gene product by the foreign promoter sequences. We propose that the level of whiG sigma factor is crucial in determining the developmental fate of hyphae.     

The main early and late promoters of Bacillus subtilis phage phi 29 form unstable open complexes with sigma A-RNA polymerase that are stabilized by DNA supercoiling.

Most Escherichia coli promoters studied so far form stable open complexes with sigma 70-RNA polymerase which have relatively long half-lives and, therefore, are resistant to a competitor challenge. A few exceptions are nevertheless known. The analysis of a number of promoters in Bacillus subtilis has suggested that the instability of open complexes formed by the vegetative sigma A-RNA polymerase may be a more general phenomenon than in Escherichia coli. We show that the main early and late promoters from the Bacillus subtilis phage phi 29 form unstable open complexes that are stabilized either by the formation of the first phosphodiester bond between the initiating nucleoside triphosphates or by DNA supercoiling. The functional characteristics of these two strong promoters suggest that they are not optimized for a tight and stable RNA polymerase binding. Their high activity is probably the consequence of the efficiency of further steps leading to the formation of an elongation complex.     

The program of protein synthesis during sporulation in Bacillus subtilis

The program of protein synthesis was examined during sporulation in Bacillus subtilis as an index of the control of gene expression. At various stages of growth and spore formation, cells of B. subtilis were pulse-labeled with ³⁵S-methionine. Protein was extracted from the radioactively labeled bacteria and then subjected to high resolution one-dimensional and two-dimensional slab gel electrophoresis. We report that sporulating cells restricted or “turned off” the synthesis of certain polypeptides characteristic of the vegetative phase of growth. In certain cases, this “turn off” was prevented in a mutant (SpoOa-5NA) blocked at the first stage of spore formation. Sporulating bacteria also elaborated new polypeptide species that could not be detected in vegetatively growing cells or in cells of the asporogenous mutant SpoOa-5NA in sporulation medium. The synthesis of these sporulation-specific proteins was “turned on” in a temporally defined sequence throughout the period of spore formation. Spore coat protein, for example, was first synthesized at 4 hr after the onset of sporulation, the time at which refractile prespores appeared. Certain sporulation-specific polypeptides including the coat protein were among the most actively produced polypeptides in sporulating cells.     

Characterization of a sporulation gene, spoIVA, involved in spore coat morphogenesis in Bacillus subtilis.

Mutations in the spoIVA locus of Bacillus subtilis abolish cortex synthesis and interfere with the synthesis and assembly of the spore coat. We have characterized the cloned spoIVA locus in terms of its physical structure and regulation during sporulation. The locus contains a single gene capable of encoding an acidic protein of 492 amino acids (molecular weight, 55,174). The gene is transcribed from a sigma E-dependent promoter soon after the formation of the spore septum. A genetic test indicated that expression of spoIVA is only necessary in the mother cell compartment for the formation of a mature spore. This, together with the phenotypic properties of spoIVA mutations, would be in accord with the hypothesis that sigma E is only active after septation and in the mother cell compartment.