What is a sigma factor? How does elevated temperature induce transcription of heat shock proteins in bacteria via a sigma factor?

Question

What will be an ideal response?

Answer 1

Sigma factor (?) is a protein that helps RNA polymerase recognize and bind to a promoter region of DNA. Regulation of the heat-shock sigma factor sigma H (also called RpoH) in E. coli illustrates how the synthesis of a sigma factor can be controlled at the level of translation and degradation. At 30°C, mRNA from the sigma H gene (rpoH) adopts a secondary structure that buries the ribosome binding site, so rpoH mRNA is poorly translated. Inappropriate expression of heat-shock genes at 30°C is prevented by the DnaK-DnaJ-GrpE chaperone system, which interacts with sigma H and shuttles it to various proteases for digestion.

To get the process of transcription going, the RNA polymerase needs to find the right part of the DNA helix to start with. Excessive heat, above 42°C for E. coli, causes proteins to denature and membrane structure to deteriorate. All cells subjected to heat above their comfort zone (optimal growth range) will express a set of proteins called heat-shock proteins. These proteins include chaperones that refold damaged proteins, and a variety of other proteins that affect DNA and membrane integrity. The transcription of many E. coli heat-shock genes requires sigma H. So, one of the first consequences of growth at elevated temperature is an increase in the amount of sigma H present in the cell.

A sudden rise in temperature melts the secondary structure of the mRNA from rpoH, exposing the ribosome-binding site, allowing translation to occur more readily. Thus, heat shock increases sigma H synthesis, which in turn increases transcription of the heat-shock genes whose products include chaperones and proteases. At 42°C, misfolded cytoplasmic proteins siphon off the chaperone trio and release sigma H to direct transcription of the heat-shock genes.

The sigma factor attaches to the RNA polymerase and it is actually what recognizes the promoter sequence which is located upstream of the gene of interest. Once it finds its match, the RNA polymerase then opens up the helix and start transcribing the mRNA to make a protein. As soon as this starts, the sigma factor is released and can go off and find another RNA polymerase to help in finding the right promoter region.