- Elongation Factor IV (EF4) is a translation factor found in nearly all eukaryotes and all sequenced prokaryotes. It was originally named LepA due to its location on the Lep operon. - EF4 functions as a back-translocase during elongation, reversing the transition catalyzed by Elongation Factor G. It preferentially binds to post-translocation ribosomal complexes. - EF4 and EF-G share structural homology and EF4 retains 55-68% amino acid identity among bacterial orthologs. EF4 binds competitively with EF-G to the ribosome.