Lec 11 - Alternative Sigma Factors

Question 1

Generally, how can the expression and activities of sfs be regulated?

Answer 1

• mRNA secondary structure
• anti sfs
• proteolysis
• anti anti sf

Question 2

How many sf does E. coli have? List them all.

Answer 2

7

• sf70 - housekeeping
• sfS - general stress factor
• sfH - heat shock
• sfF - flagella synthesis
• sfI - iron transport
• sfN - nitrogen metabolism
• sfE - extracytoplasmic stress

Question 3

What promoters does sf70 recognise?

Answer 3

-35 and -10

Question 4

How many anti-sf70s exist to sequester sf70 in stationary phase?

Answer 4

2 (don’t know which)

Question 5

At 30C, how is sfH degraded as to not stimulate transcription of heat shock genes?

Answer 5

Chaperone proteins, eg DnaK, bind to sfH that has managed to be translated and directs it to protease to be degraded

Question 6

At 42C why does DnaK and other chaperones not bind to sfH?

Answer 6

Preoccupied with binding to other unfolded native proteins in attempt to refold them. Allows sfH to be translated, bind to RNAP (holoenzyme) and initiate transcription of heat shock genes

Question 7

What promoter elements does sfS recognise?

Answer 7

-35 and -10, with slight base sequence differences to sf70. Also binds to same site on RNAP but at lower affinity

Question 8

When is sfI synthesised?

Answer 8

under low Fe conditions (detected by lack of Fur binding to Fe) and when ferric citrate is present (taken up by FecA)

Question 9

Describe the fec operon

Answer 9

Pfur. PfecI. fecIR. Pfur. PfecA. fecABCDE

Question 10

How is Fe citrate transported into the cytoplasm?

crosses 2 barriers, OM and IM

Answer 10

via FecA complex interacting with the TonB complex that provides energy needed for this uptake
from periplasm -> cytoplasm, ABC type transporter, encoded for by fecBCDE?, is used

Question 11

Why is sfN not classed as part of the sf70 family?

Answer 11

Does not spontaneously form an open complex to initiate transcription

Question 12

What are the promoter sites for sfN?

Answer 12

-24 and -12

Also binding sites @ upstream enhancer region

Question 13

What are the 2 things sfN is dependent upon to initiate transcription? (whilst describing them also try to describe the process)

Answer 13

• NAP, IHF to bend the DNA in order for the enhancer site (with attached activator tfs) to come into contact with the sfN bound to RNAP
• AAA+ transcription factors that are bound to enhancers. Upon stimulation via a N source, ATP hydrolysis of the tfs occurs and allows the conformational change of sfN to form an open complex w/ RNAP. transcription can then occur and sfN is lost