Question

3. If each sigma factor recognizes a different promoter consensus sequence, how does the cell transcribe a gene that responds to multiple conditions, where each condition utilizes a different sigma factor? What would happen to overall gene expression profiles if RpoS were artificially overexpressed and why?

Answer 1

Sigma factors are proteinaceous regulators of gene expression in prokaryotes. These proteinaceous factors allow specialized binding of the RNA polymerase on the specific promoter sequenes thus allowing expression of specific genes in the bacteria, under conditions of stress/stimulus. Especially, each sigma factor identifies a different promoter sequence in the cells. Also, the bio-activation of these sigma factors is highly dependent upon the changes in physiological conditions of the cells. For example, cells generally express a sigma factor which encodes for normal gene expression. But under condition of heat stress, sigma-45 is generally expressed which upregulates the expression of heat-shock proteins in bacteria. It is important to note here that although the induced expression of gene changes but the native levels of all the vital protein are largely maintained in the cells. This ensures that the cell is able to maintain homeostasis even under stress. Thus, stress induced binding of different sigma factors can be based upon the fact native expression of gene remains unaltered.

Similarly, if the gene expression of RpoS is artificially enhanced, that would not largely affect the overall gene expression becuase mere presence of RpoS does not imply that stress-induced genes will be up-regulated in the cells. The physical presence of stress/stimulus is mandatory for bio-activation of these strerr-modulators of sigma factors and hence their binding to appropriate promoters.