Initiation factor IF 2 binds to the alpha-sarcin loop and helix 89 of Escherichia coli 23S ribosomal RNA

During initiation of protein synthesis in bacteria, translation initiation factor IF2 is responsible for the recognition of the initiator tRNA (Met-tR NA). To perform this function, IF2 binds to the ribosome interacting with b oth 30S and 50S ribosomal subunits. Here we report the topographical locali zation of translation initiation factor IF2 on the 70S ribosome determined by base-specific chemical probing. Our results indicate that IF2 specifical ly protects from chemical modification two sites in domain V of 23S rRNA, n amely A2476 and A2478, and residues around position 2660 in domain VI, the so-called sarcin-ricin loop. These footprints are generated by IF2 regardle ss of the presence of fMet-tRNA, GTP, mRNA, and IF1. IF2 causes no specific protection of 16S rRNA. We observe a decreased reactivity of residues A141 8 and A1483, which is an indication that the initiation factor has a tighte ning effect on the association of ribosomal subunits. This result, confirme d by sucrose density gradient analysis, seems to be a universally conserved property of IF2.