During initiation of protein synthesis in bacteria, translation
initiation factor IF2 is responsible for the recognition of
the initiator tRNA (fMet-tRNA). To perform this function, IF2
binds to the ribosome interacting with both 30S and 50S ribosomal
subunits. Here we report the topographical localization of
translation initiation factor IF2 on the 70S ribosome determined
by base-specific chemical probing. Our results indicate that
IF2 specifically protects from chemical modification two sites
in domain V of 23S rRNA, namely A2476 and A2478, and residues
around position 2660 in domain VI, the so-called sarcin-ricin
loop. These footprints are generated by IF2 regardless 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 A1418 and A1483, which is an indication that the initiation
factor has a tightening effect on the association of ribosomal subunits.
This result, confirmed by sucrose density gradient analysis, seems to
be a universally conserved property of IF2.