To extend elongation into the transcription unit, a kinase complex called P-TEFb is required. P-TEFb contains the CDK9 kinase, which is a member of the kinase family that controls the cell cycle. P-TEFb acts on the CTD to phosphorylate it further on serine 2 of the heptapeptide repeat.

TLS Online TPP Program

#Id: 6415

#Unit 2. Cellular Organization

RNA Polymerase don’t get dissociated with one hairpin which is 2:3

TLS Online TPP Program

#Id: 6416

#Unit 2. Cellular Organization

Transcription is initiated at the beginning of the 162 nucleotide mRNA leader encoded by a DNA region called trpL

TLS Online TPP Program

#Id: 6417

#Unit 2. Cellular Organization

The trp operon is controlled by repression and attenuation, providing a two-stage response to progressively more stringent tryptophan starvation. But attenuation alone can provide robust regulation: other amino acid operons such as leu and his rely entirely on attenuation for their control. In the case of the leucine operon, its leader peptide has four adjacent leucine codons, and the histidine operon leader peptide has seven histidine codons in a row.

TLS Online TPP Program

#Id: 6418

#Unit 2. Cellular Organization

The promoter of the araBAD operon from E. coli is activated in the presence of arabinose and the absence of glucose and directs expression of genes encoding enzymes required for arabinose metabolism. 

Unlike the cases of lac and gal genes, where a repressor and an activator work together, here two activators work together: AraC and CAP. 

TLS Online TPP Program

#Id: 6419

#Unit 2. Cellular Organization

When arabinose is present, AraC binds that sugar and adopts a configuration that allows it to bind DNA as a dimer to the adjacent half-sites, araI1 and araI2). 

Just upstream of these is a CAP site: in the absence of glucose, CAP binds here and helps activation.

In the absence of arabinose, the araBAD genes are not expressed. This is because when not bound to arabinose, AraC adopts a different conformation and binds DNA in a different way: one monomer still binds the araI1 site, but the other monomer binds a distant half-site called araO2.

TLS Online TPP Program

#Id: 6420

#Unit 2. Cellular Organization