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Advanced Microbial Physiology

Advanced Microbial Physiology . Lecture 3 Sigma Factors. Three Types of Gene Regulations. Transcriptional Control Attenuation Controls Global Control Networks. TRANSCRIPTIONAL CONTROL. Definitions Structural gene Promoter Operator Operon and polycistronic Regulatory gene Regulon

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Advanced Microbial Physiology

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  1. Advanced Microbial Physiology Lecture 3 Sigma Factors

  2. Three Types of Gene Regulations • Transcriptional Control • Attenuation Controls • Global Control Networks

  3. TRANSCRIPTIONAL CONTROL • Definitions • Structural gene • Promoter • Operator • Operon and polycistronic • Regulatory gene • Regulon • Inducible • Repressible

  4. RNA SYNTHESIS • Process is the same for synthesis of all three types of RNA • Catalyzed by RNA polymerase • Transcription consists of three main steps: • initiation • elongation • termination

  5. Bacterial RNA polymerase • Responsible for synthesis of all 3 types of RNA species • Huge enzyme (400 kD) made of five subunits: • 2 a subunits • 1 b subunit • 1 b’ subunit • 1  factor holoenzyme core enzyme

  6. Transcription Initiation

  7. Promoter structure (σ70 type)

  8. Elongation (polymerization)

  9. Sigma Factors • As the regulon of a single sigma factor can be comprised of hundreds of genes, sigma factors provide effective mechanisms for simultaneously regulating large numbers of prokaryotic genes. • In some cases, the genes comprising a sigma factor regulon have a clearly defined primary function (eg, sporulation). • In others, the genes comprising a regulon contribute to multiple functions such as general stress response.

  10. Sigma Factors in Bacillus subtilis • Sigma subunit of RNAP determines the specificity of promoter utilization. • There are at least 10 different sigma factors in B. subtilis, each of which directs RNAP to a different set of promoters. • Most sigma factors make sequence-specific contacts at -10 and -35 regions upstream of regulated genes or operons.

  11. Sigma Factors involved in Virulence • Virulence genes encode proteins whose functions are essential for the bacterium to effectively establish an infection in a host organism. • Virulence-associated genes can contribute to bacterial survival in the environment or to survival in the host. • These two classes of genes can be regulated by a series of alternative sigma factors.

  12. Alternative Sigma Factors Classification • Alternative sigma factors can be classified into two structurally unrelated families: • σ70 and σ54 • Although no sequence conservation exists between σ70 and σ54–like family members, both types bind to core RNA polymerase. • Promoter structures recognized by σ54–RNAP differ from those recognized by σ70 –RNAP. • σ54–RNAP recognizes -24 and -12 • σ70 –RNAPrecognizes -35 and -10.

  13. Stress Response Alternative Sigma Factors

  14. Stress Response Alternative Sigma Factors • In B. subtilis, sigB, which encodes σB, is the 7th open reading frame in an operon containing 8 genes involved in σB regulation. • All 8 genes are cotranscribed from a housekeeping sigma factor (σA)-dependent promoter (PA). • A σB –dependent promoter (PB), located upstream of rsbV, is responsible for enhanced transcription of the four downstream genes in the sigB operon under conditions that stimulate σB activity.

  15. Sigma B • A B. anthracis sigB mutant strain is virulence attenuated, producing less than half the mortality of the parent strain in the mouse model of anthrax. • In S. aureus strains, as many as 251 genes have been identified as being σB regulated, including several genes encoding proteins involved in synthesis of capsular polysaccharides.

  16. Sigma N • σN of P. syringae controls hrp gene expression and influences virulence. • Regulation occurs via a short regulatory cascade, wherein σN and its enhancer-binding proteins HrpR and HrpS direct transcription of hrpL, the product of which is the alternative sigma factor required for expression of the hrp and avr genes.

  17. Mycobacterial ECF Sigma Factors • Members of the extracytoplasmic function (ECF) subfamily of σ70 sigma factors regulate functions related to sensing and responding to changes in the bacterial periplasma and extracellular environment. • Mycobacterium tuberculosis has 13 recognized sigma factors; among these, 10 are ECF sigma factors. • At least six M. tuberculosis sigma factors affect virulence, including the primary sigma factor, σF, and four ECF sigma factors, σC, σD, σE and σH.

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