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Microbiology (Biological Sciences)
Nicolas Vozza

1 ALTERNATIVE σ FACTORS They direct RNAP to initiate transcription at specific promoters to allow differential expression of a particular subset of genes. First described for Bacillus subtilis & phages SP01/SP82 Now ten different families. And a common feature in all bacteria e.g. – E. coli – 7 factors The specificity of RNAP for promoter sequences can be modified by different σ factors. Some are absolutely specific for their own promoter sequences. Others have promoter specificities that overlap with other sigma factors. Need to know some sigma factors. Sigma 70 most abundant. Phage have different sigma factors for different life stages. Sigma 70 recognizes early genes. Makes sigma factors for middle/late genes. Sigma S is similar to sigma 70 and used for stationary phase expression. Don’t know many sigma factors. Sigma factors have evolutionary tree that is devided into groups. 2 Questions: 1. How many sigma factors can bacteria have? 2. What does it depend on? The different conditions that the bacteria can live under. Presence of a certain substrate. Stresses. Changes in life cycle. 3. What does it help with? M. genitalium: smallest genome. One sigma factor. S. coelicolor: soil bacteria with large genome. Over 60 sigma factors. This gives flexibility in promoter use. And depends upon cellular concentrations. Types of σ Regulation 1. Concentration A. Synthesis B. Protein stability 2. Activity modulated by modification A. Cleavage of protein sequence B. Interaction with inhibitors (anti sigma factors) 3 Examples σS – Stationary Phase When cells stop growing exponentially, many genes no longer needed. Instead, slow growth stationary phase genes are turned on. 3 Many of these genes are transcribed by σS Encoded by rpoS. Sometimes called sigma 38. Not the only stationary phase–specific alternative σ factor, but the major one. Given that exponential bacterial growth is not normal… (in real conditions). Perhaps high levels of sigma S is normal Also expressed under cell stress At exponential growth: Sigma S = 0/cell. Sigma 70 = 1000/cell. At stationary phase: Sigma 70 = 300/cell σS shares many promoter specificities with σ70. Some promoters are recognized by both σ factors. Unusual since most alternative sigma factors are specific for a class of genes. Sigma S controls many genes. Regulation of σS (rpoS) mainly occurs at level of transcription, but also at post-translational stages and protein stability 4 σH (σ32) and σE (σ24) o When E. coli growth temperatures reach > 40 C, new genes are expressed. Heat shock resonse. Or thermotolerance. This involves the expression of approx. 20 new proteins. Heat Shock (HS) is a universal phenomenon since organisms from bacteria to mammals do it, and express many of the same proteins. HS genes encode Proteases and a family of proteins called chaperones which refold thermally denatured proteins. σH recognizes a completely new promoter sequence, so all HS genes require σH and core RNAP. These genes are expressed at a basal level at normal temperatures so low levels of sigma 32 (sigma H) are expressed. σH gene is recognized by σ70 at 40 C. Levels of σH increase 20 fold during HS. T 1/2= 1 min therefore this is a fast response both increasing and decreasing. Short half life means quick disappearance when stimulus is not present. 5 This is mostly due to an increase in translational efficiency; the mRNA is easier to translate at higher temps (less folding energy to get sigma H secondary structure). Sigma H half life also increases at higher temperatures. When temperatures rise to 50 C, HS is more extreme, and continuous synthesis of HS proteins is required. Extreme HS Rarely found at normal temperatures, σH & σE become o abundant above 40 C. σE does not recognize σH or σ70 promoters (these genes are for thermotolerance). But instead indir
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