Question

Which is incorrect about the initiation of translation?

Select one:

a. 30S subunit binds to mRNA first by recognizing the Shine-Dalgarno sequence on mRNA.

b. In bacteria, most of the starting Met codon is specifically for N-formyl methionine (fMet).

c. After fMet-loaded tRNA is bound, the 50S subunit binds to make the complete translational complex.

d. A complete ribosome complex migrates on an mRNA until it finds the Shine-Dalgarno sequence.

e. All of these

Answer 1

d. A complete ribosome complex migrates on an mRNA until it finds the Shine-Dalgarno sequence.

In prokaryotes, translation initiation requires three small proteins called initiation factors IF1,IF2 and IF3. As one of the first IF3 readily binds to the small ribosomal subunit, its presence block the large and small subunits of ribosomes from prematurely associating. IF3 facilitates the binding of mRNA to the small subunit of the ribosome. The binding occurs just 4-8 bases upstream of the AUG start , where a concensus sequence called Shine-Dalgarno sequence in the mRNA anneals near the end of the 16s ribosomal RNA in the small ribosomal subunit. IF1 binds to the small subunit at a location called the A site where incoming tRNA is normally bind.The first tRNA called the N-formyl methionyl-tRNA (fMet) thus enters another site called P-site. The initiator tRNA has been escorted to the P site by IF2 which is bound to GTP, a high energy molecule similar to ATP. The anticodon of the tRNA is complementary to the AUG start codon. Once initiator tRNA is in place , IF3 is released. With the loss of IF3 the 50s subunit can dock on the 30s subunit. The docking of 50s subunit triggers the hydrolysis of GTP on IF2 and the susequent release of the intiation factors, the ribosome is now ready for elongation.