Untitled

Abstract

The processes of association and dissociation of ribosomal subunits are of great importance for the protein biosynthesis. The mechanistic details of these processes, however, are not well known. In bacteria, upon translation termination, ribosome dissociates into subunits which is necessary for its further involvement into new initiation step. The dissociated state of ribosome is maintained by initiation factor 3 (IF3) which binds to free small subunits and prevents their premature association with the large subunits. In this work, we have exchanged IF3 in E.coli cells by its ortholog from Saccharomyces cerevisiae mitochondria (Aim23p) and showed that yeast protein cannot functionally substitute the bacterial one and is even slightly toxic for bacterial cells. 49 binds to the small subunit in order to keep it apart from the large one . This 50 stage is, in fact, the very first stage of the translation initiation process; 30S•IF3 complex 51 becomes the basis for the full-size initiatory complex formation which includes Shine-Dalgarno 52 sequence of mRNA, initiator tRNA, and initiation factors 1 and 2. It is worth mentioning that 53 anti-association activity of IF3 is definitely of passive mode: it does not promote dissociation of 54 the ribosome into subunits but instead binds to free small subunit and prevents its re-association 55 with the large one (Gualerzi et al. 1977) (Gottleib et al. 1975). 56The exact mechanism of ribosome dissociation into subunits remains not clear. This is 57 due to methodological complications of studying this fast and dynamic process. In kinetic study, 58 a model was proposed that assumed the existence of several consecutive conformations of 70S 59 ribosome in course of its dissociation; IF3 was hypothesized to be a potential effector of 60 corresponding conformational changes which could shift the equilibria between different states 61 of dissociating ribosome (Goss et al. 1980). It can be assumed that these conformations might be 62 characterized by different sedimentation coefficients, less than 70S but probably more than 50S.