Sigma factor

Sigma factors ( σ ) factors are bacterial proteins which are necessary for the initiation of transcription.

Usually sigma factors in the cell are linked to the RNA polymerase. A complete bacterial polymerase bonded sigma factor is often referred to as a holoenzyme. It consists of six subunits ( α2ββ'ωσ ). The minimum or core enzyme, however, is not bound to the σ - subunit. In other contexts, the importance of holoenzyme, however, is often another.

Sigma factors have a high affinity for the Pribnow box and the -35 sequence of the promoter. Thus, it increases the probability of binding polymerase holoenzyme to the start site of the open reading frame of the DNA. Without sigma factor capable of binding to the DNA, the RNA polymerase but does not occur for transcription. If the polymerase holoenzyme is the open complex and transcription begins, the sigma- factor is cleaved off.

Depending on the environmental conditions expressing bacteria usually several different sigma factors, which generally have different promoter specificities. In this way, the transcription of specific genes is taught to adapt to the environmental conditions. There are two known classes of sigma factors. A class with many representatives has homologies to factor sigma 70 bacterial species Escherichia coli. A smaller family - in most bacteria with only a single representative - is homologous to the E. coli sigma factor - 54th This greatly both structurally and in the mechanism of initiation of transcription is different from the sigma- 70 - family. Sigma factors are characterized by their molecular weight. The σ70 factor, for example, describes the sigma factor with a molecular mass of 70 kDa.

Sigma factors from E. coli:

• Sigma 70 is the household ( housekeeping ) sigma factor of E. coli, which initiates the transcription of those genes whose gene products are required under normal environmental conditions.

• Sigma 32 is the heat-shock sigma factor in E. coli, which is encoded by rpoH gene. If the temperature in the cell, the factor is synthesized increased. Due to the increased concentration of sigma-32 in the cell, the protein binds with high probability to the polymerase core enzyme. This heat shock proteins are expressed that help the cell to survive the elevated temperatures. They include, for example, chaperones, proteases, and DNA repair enzymes.

• Sigma 54 is encoded by the gene and produced at rpoN nitrogen deficiency. Sigma 54 holoenzyme activated inter alia, the expression of glutamine synthetase, which is the key enzyme for nitrogen assimilation.

• Sigma 38, sigma- factor of the overall response to stress in the E. coli. Stress factors such as carbon starvation, amino acid starvation or ketoacidosis Sigma is upregulated 38 expression. This is for example the case when a bacterial culture is in the stationary phase. Under sigma 38 proteins are expressed, protect the cell against environmental influences.