Advanced Microbial Physiology

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Advanced Microbial Physiology

• Lecture 3
• Sigma Factors

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Three Types of Gene Regulations

• Transcriptional Control
• Attenuation Controls
• Global Control Networks

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TRANSCRIPTIONAL CONTROL

• Definitions
• Structural gene
• Promoter
• Operator
• Operon and polycistronic
• Regulatory gene
• Regulon
• Inducible
• Repressible

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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

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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
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Transcription Initiation
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Promoter structure (s70 type)
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Elongation (polymerization)
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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.

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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.

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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.

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Alternative Sigma Factors Classification

• Alternative sigma factors can be classified into
  two structurally unrelated families
• s70 and s54
• Although no sequence conservation exists between
  s70 and s54like family members, both types bind
  to core RNA polymerase.
• Promoter structures recognized by s54RNAP differ
  from those recognized by s70 RNAP.
• s54RNAP recognizes -24 and -12
• s70 RNAP recognizes -35 and -10.

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Stress Response Alternative Sigma Factors
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Stress Response Alternative Sigma Factors

• In B. subtilis, sigB, which encodes sB, is the
  7th open reading frame in an operon containing 8
  genes involved in sB regulation.
• All 8 genes are cotranscribed from a housekeeping
  sigma factor (sA)-dependent promoter (PA).
• A sB 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 sB
  activity.

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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 sB regulated, including
  several genes encoding proteins involved in
  synthesis of capsular polysaccharides.

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Sigma N

• sN of P. syringae controls hrp gene expression
  and influences virulence.
• Regulation occurs via a short regulatory cascade,
  wherein sN 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.

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Mycobacterial ECF Sigma Factors

• Members of the extracytoplasmic function (ECF)
  subfamily of s70 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,
  sF, and four ECF sigma factors, sC, sD, sE and sH.

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