Question

Protein factors (IFs, EFs, and eRFs) play critical roles in each of the steps in translation of a message by the ribosome. Which of the following general statements pertaining to their functions and mechanisms of action is/are true?

	A.	Some of these factors bind tRNA and facilitate their binding to the appropriate sites in the ribosome.
	B.	Some of these factors bind GTP and rely on their hydrolysis to GDP to signal the correct interactions.
	C.	These factors are involved in initiation, elongation, and termination of translation.
	D.	Two of the above.
	E.	All of the above.

Answer 1

All of the above statements regarding the initiation factors (IFs), elongation factors (EFs) and eukaryotic release factors (eRFs) that play an important role in initiation, elongation and termination steps in translation. Prokaryotic IF1 associates with 30S ribosomal subunit in A site preventing aminoacy-tRNA from entering.It modulaes IF2 binding to ribosome by increasing its affinity. It may also prevent 50S subunit from binding to 30S subunit to form 70S subunit. It is a homolog of eukaryotic initiation factor (eIF) eIF1A. IF2 and GTP complex binds to initiator tRNA and controls the entry of that tRNA into ribosome. After transfering the initiator tRNA to P site in ribosome and binding of 50S subunit, GTP is hydrolysed to GDP and Pi causing a conformational change in IF2 which causes IF2 to release. It is homolog of eIF5B. IF3 is required by the small subunit to form initiation complex, but is released in order to facilitate binding of 50S subunit. It also allows rapid interaction between codon anticodon for the initiator tRNA to bind quickly. In the next step of translation elongation factors play an important role. Prokaryotic elongation factor EF-Tu mediates the entry of aminoacyl-tRNA into the free site on ribosome. Its eukaryotic homolog is eEF1 subunit alpha. EF-Ts acts as guanine nucleotide exchange factor for EF-Tu catalysing release of GDP from EF-Tu. Its eukaryotic homolog is eEF1 subunit beta and gamma. EF-G (eukaryotic homolog eEF2) catalyzes translocation of the tRNA and mRNA down the ribosome at the end of each round of polypeptide elongation. EF-P (eukaryotic homolog EIF5A) stimulates peptide bond formation and resolves stalls. In the termination step release factors plat an important role for the identification of stop codons and release of polypeptide chain. RF1 recognizes UAA and UAG stop codons whereas RF2 recognizes UAA nad UGA stop codons. RF3 are class II release factors that aid in rapid release of RF1/2 in GTP dependent manner.