Sigma factor

Sigma 70 domains

PDB 1ku2 EBI

PDB 1sig EBI

PDB 1rp3 EBI

PDB 1tty EBI

PDB 2h27 EBI

Sigma factor is a protein that plays a crucial role in the process of transcription in bacteria. It is a type of transcription factor that enables the RNA polymerase to recognize and bind to specific promoter regions on the DNA, initiating the transcription of genes. Sigma factors are essential for the expression of a wide array of genes, including those involved in cell growth, stress response, and metabolism.

Function

The primary function of sigma factors is to direct the RNA polymerase to specific promoter sequences ahead of genes, ensuring that the right genes are transcribed at the right time. Different sigma factors recognize different promoter sequences, allowing bacteria to respond to environmental changes by switching which sigma factor is predominant. This switch changes the pattern of gene expression, enabling the bacteria to adapt to new conditions.

Types of Sigma Factors

Sigma factors can be broadly classified into two groups: the primary sigma factor and alternative sigma factors. The primary sigma factor is responsible for the transcription of essential housekeeping genes under normal growth conditions. In contrast, alternative sigma factors are activated under specific environmental or cellular conditions, such as heat shock or nutrient limitation, and direct the transcription of genes necessary for the response to these conditions.

Primary Sigma Factor

The primary sigma factor in Escherichia coli is sigma 70 (σ70). It is involved in the transcription of a large number of genes required for growth and survival in standard conditions.

Alternative Sigma Factors

Alternative sigma factors include sigma 32 (σ32), which is involved in the heat shock response, and sigma 54 (σ54), which is associated with the transcription of genes involved in nitrogen metabolism. Other examples include sigma S (σS), important for the general stress response, and sigma F (σF), which plays a role in sporulation in Bacillus subtilis.

Regulation

The activity of sigma factors is tightly regulated by the cell to ensure that gene expression is adapted to environmental conditions. Regulation can occur at multiple levels, including the synthesis and degradation of sigma factors, competition between sigma factors for binding to RNA polymerase, and the sequestration of sigma factors by anti-sigma factors.

Role in Pathogenesis

Some sigma factors are involved in the pathogenesis of bacterial infections. For example, sigma factors in pathogenic bacteria can activate the expression of virulence genes, enabling the bacteria to invade host tissues, evade the immune system, or produce toxins.

Biotechnological Applications

Due to their role in gene expression, sigma factors are of interest in biotechnology for the development of novel antibiotics and the engineering of bacteria for the production of recombinant proteins, biofuels, and other valuable products.