Substitution of Sigma Factor and Control of Transcription

We earlier discussed the role of sigma (σ) factor in recognition of promoter site. Therefore, whenever more than one sigma factors are available for the same core enzyme, they can be used for recognition of different promoters. Substitution of sigma factor has been shown to result in switching 'on' certain 'genes in E. coli. In some other bacteria, it leads to change in the style of life from vegetative phase to sporulation phase. Substitution of sigma factors is also used for regulation of gene expression in bacteriophages during transition from lysogenic phase to lytic phase (see next main topic for details).

Multiple sigma factors in E. coli
In E. coli, no change from vegetative phase to sporulation phase occurs, and therefore for a long time it was believed that it has a single sigma factor, a70. However, in recent years, new sigma factors in E. coli have been discovered, which operate under conditions of heat shock (σ32 or σH), nitrogen starvation54 or σN) or for chemotaxis and flagellar structure (σ28 or σ11). The details of these sigma factors are given in Table 35.1.


Sporulation in bacteria
In bacteria like Bacillus subtilis, at the end of the vegetative phase, nutrients in the medium get depleted and a series of morphological changes are initiated. This involves segregation of a genome at one end of the cell, where eventually it gets surrounded by a tough spore coat. This process takes about eight hours and involves drastic changes in the biosynthetic activities of the bacterium. Synthesis of RNA continues throughout sporulation and its inhibition brings the process to an end. During this process some of the genes functioning during vegetative phase are turned off, although most genes still function. But a large number of genes specific for sporulation are expressed only during this period. At the end of sporulation 40% of mRNA is specific to sporulation.

Changes in the initiation specificities of RNA polymerase by replacement of sigma factors during sporulation in Bacillus subtilis
Fig. 35.19. Changes in the initiation specificities of RNA polymerase by replacement of sigma factors during sporulation in Bacillus subtilis.
Genes specific for sporulation can be identified by spcr mutations, which will block sporulation. These mutants may be spo 0, where sporulation does not start at all; others could be spo II, etc. where later stages are affected. These genes either synthesize enzymes or structural proteins or are involved in transition from vegetative phase to sporulation.

Some of the changes which occur during change of life style from vegetative to sporulating phase are now known and have been studied to indicate that alterations in RNA polymerase are involved in the initiation of sporulation. In RNA polymerase of B. subtilis, atleast three sigma factors namely σ43 (43,000 daltons), σ37 (37,000 daltons) and σ29 (29,000 daltons) have been identified, σ43 is involved in transcription during vegetative phase, while σ37, and σ29 are involved during sporulation (Fig. 35.19). At the beginning of sporulation σ43 is replaced by σ37, perhaps due to the activity of any of the five spo 0 genes, which either removes a43 or modifies the core enzyme. The displaced a43 can be recovered from sporulating cells.
Therefore, α2ββ'σ37 initiates the transcription of gene needed for sporulation. About four hours after sporulation, another protein σ29 appears and RNA polymerase α2ββ'σ29 functions for transcription (Fig. 35.19). Several other sigma factors have been isolated and characterized in B. subtilis. Thus, it is obvious that selective expression of genes can be caused by changes in sigma factor of RNA polymerase enzyme.
Changes in the initiation specificities of RNA polymerase by replacement of sigma factors during sporulation in Bacillus subtilis
Fig. 35.19. Changes in the initiation specificities of RNA polymerase by replacement of sigma factors during sporulation in Bacillus subtilis.