The 3¹⁰ helix has 3 residues per tum and contains 10 atoms between the hydrogen bond donor and acceptor, hence its name.

TLS Online TPP Program

#Id: 2329

#Unit 3. Fundamental Processes

Once DNA has been melted, the trajectory of the individual strands within the enzyme is no longer constrained by the rigidity of the double helix, allowing DNA to make its 90° turn at the active site.

TLS Online TPP Program

#Id: 2330

#Unit 3. Fundamental Processes

There are RNA polymerase crystal structures that provide considerable insight into how the enzyme retains contact with its substrate while breaking and remaking bonds in the process of the nucleotide addition cycle and undergoing translocation by a Brownian ratchet mechanism.

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.