During the transition to the open complex, the conformation of both the RNA polymerase and the DNA change. 

1) 90° bend in the DNA, which allows the template strand to approach the active site of the enzyme; 

2) strand opening of the promoter DNA between -11 and +3 with respect to the transcription start site; 

3) the promoter DNA scrunches into the active channel forming the transcription bubble; and 

4) closing of the jaws of the enzyme to encircle the section of the promoter downstream section of the transcription start site.

TLS Online TPP Program

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#Unit 3. Fundamental Processes

TLS Online TPP Program

#Id: 7089

#Unit 3. Fundamental Processes

IF1 prevents tRNAs from binding to the portion of the small subunit that will become part of the A-site.

†IF2 is a GTPase that interacts with three key components of the initiation machinery: the small subunit, IF1, and the charged initiator tRNA (fMet-tRNAi f-Met). By interacting with these components, IF2 facilitates the association of fMet-tRNAi f-Met with the small subunit and prevents other charged tRNAs from associating with the small subunit.

† IF3 binds to the small subunit and blocks it from reassociating with a large subunit. Because initiation requires a free small subunit, the binding of IF3 is critical for a new cycle of translation. 

TLS Online TPP Program

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#Unit 3. Fundamental Processes

TLS Online TPP Program

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#Unit 3. Fundamental Processes

binding by a ribosome, many prokaryotic ORFs contain a short sequence upstream (on the 5’ side) of the start codon called the ribosome binding site (RBS). This element is also referred to as a polypurine stretch Shine–Dalgarno sequence.

TLS Online TPP Program

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#Unit 3. Fundamental Processes

It is complementary to a highly conserved sequence close to the 3’ end of 16S rRNA.

The rRNA sequence is the hexamer:

3’. . . U C C U C C . . . 5’

TLS Online TPP Program

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#Unit 3. Fundamental Processes

 5’-AUGA-3’ is sequence between overlapping ORFs is known as translational coupling.