Abstract
The translation initiation factor aIF2 of the crenarchaeon Sulfolobus solfataricus (Sso) recruits initiator tRNA to the ribosome and stabilizes mRNAs by binding via the gamma-subunit to their 5'-triphosphate end. It has been hypothesized that the latter occurs predominantly during unfavorable growth conditions, and that aIF2 or aIF2-gamma is released on relief of nutrient stress to enable in particular anew translation of leaderless mRNAs. As leaderless mRNAs are prevalent in Sso and aIF2-gamma bound to the 5'-end of a leaderless RNA inhibited ribosome binding in vitro, we aimed at elucidating the mechanism underlying aIF2/aIF2-gamma recycling from mRNAs. We have identified a protein termed Trf (translation recovery factor) that co-purified with trimeric aIF2 during outgrowth of cells from prolonged stationary phase. Subsequent in vitro studies revealed that Trf triggers the release of trimeric aIF2 from RNA, and that Trf directly interacts with the aIF2-gamma subunit. The importance of Trf is further underscored by an impaired protein synthesis during outgrowth from stationary phase in a Sso trf deletion mutant.