What are initiation factors and give examples in bacteria and eukaryotes.

• A. Bacteria IF1/IF2/IF3; Eukaryotes eIF2, eIF4E, eIF4G
• B. Only bacterial initiation factors exist; eukaryotes use cap-binding proteins without factors
• C. Initiation factors are enzymes that catalyze initiation
• D. Initiation factors are ribosomal RNAs only

Answer: (A)

Initiation factors are helper proteins that coordinate the first steps of translation, guiding the ribosome to the mRNA and ensuring the start codon is read correctly. This is why the examples span both bacteria and eukaryotes.

In bacteria, a small set of factors orchestrates the start of protein synthesis. IF1 helps position things and prevents unwanted tRNA binding to the A site, IF2 is a GTPase that brings the initiator tRNA (fMet-tRNA) to the P site, and IF3 helps with start-site selection and keeps the small and large subunits from prematurely joining. Together, they set up a correct initiation complex and pave the way for the large subunit to join.

In eukaryotes, initiation relies on a group of initiation factors known as eIFs. A key player is eIF2, which forms a GTP-bound complex with the initiator methionyl-tRNAi and delivers it to the small ribosomal subunit. Cap recognition and recruitment are handled by a cap-binding module where eIF4E binds the 5' cap and eIF4G (with eIF4A) acts as a scaffold and helicase, facilitating scanning for the start codon. This combination of factors ensures proper assembly of the ribosome on the mRNA and accurate initiation.

The other statements misstate what initiation factors are or where they come from. Initiation factors are not enzymes that catalyze initiation, nor are they limited to bacteria or solely ribosomal RNAs; and eukaryotes do use cap-binding features as part of a broader set of initiation factors rather than operating without them.