As the transition from G1 to S phase begins, CDK/cyclins recruit cdc45 and the GINS complex to the MCM helicase, which then becomes known as the CMG complex (Cdc45-MCM-GINS)

TLS Online TPP Program

#Id: 2331

#Unit 3. Fundamental Processes

Unlike the other elements within the promoter, the UP-element is not recognized by s but is recognized by a carboxy-terminal domain of the subunit, called the aCTD .

TLS Online TPP Program

#Id: 2332

#Unit 3. Fundamental Processes

The aCTD is connected to the aNTD by a flexible linker. 

TLS Online TPP Program

#Id: 2333

#Unit 3. Fundamental Processes

Thus, although the aNTD is embedded in the body of the enzyme, the aCTD can reach the upstream element and can do so even when that element is not located immediately adjacent to the –35 region, but further upstream.

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.