transversions, which are pyrimidine-to-purine and purine-to-pyrimidine substitutions, such as T to G or A and A to C or T.

TLS Online TPP Program

#Id: 2334

#Unit 3. Fundamental Processes

The E. coli rrn  promoters are also regulated by a pair of small molecules: the initiating NTP (the iNTP) and an alarmone,  guanosine 5 -diphosphate 3-diphosphate (ppGpp). 

1. An abundance of iNTP indicates that the concentration of  nucleotides is high, and therefore it is appropriate to synthesize plenty of rRNA. 

2. Accordingly, iNTP stabilizes the open promoter complex, stimulating transcription. 

3. On the other hand, when cells are starved for amino acids, protein synthesis cannot occur readily and the need for ribosomes (and rRNA) decreases. 

TLS Online TPP Program

#Id: 2335

#Unit 3. Fundamental Processes

A ribosome-associated protein called RelA receives the “alarm” and produces the “alarmone” ppGpp, which destabilizes open promoter complexes whose lifetimes are normally short, thus inhibiting Transcription.

TLS Online TPP Program

#Id: 2336

#Unit 3. Fundamental Processes

The protein DskA  also plays an important role. It binds to RNA polymerase and reduces the lifetimes of the rrn open promoters to a level at which they are responsive to changes in iNTP and ppGpp concentrations. 

TLS Online TPP Program

#Id: 2337

#Unit 3. Fundamental Processes

DskA is required for the regulation of rrn  transcription by these two small molecules. Indeed, rrn  transcription is insensitive to iNTP and ppGpp in mutants lacking DskA.

TLS Online TPP Program

#Id: 2338

#Unit 3. Fundamental Processes

In E. coli, rRNA synthesis from the seven rRNA operons responds to cellular growth rate and to changes in the availability of crucial nutrients, particularly amino acids. The regulation coordinated with amino acid concentrations is known as the stringent response.

TLS Online TPP Program

#Id: 2339

#Unit 3. Fundamental Processes

Amino acid starvation leads to the binding of uncharged tRNAs to the ribosomal A site; this triggers a sequence of events that begins with the binding of an enzyme called stringent factor (RelA protein) to the ribosome.