Dear Editor,Recently,a novel elongation factor,termed EF4,was identified in E.coli and shown to catalyze the back translocation of the E-and P-tRNAs (tRNA at E and P sites) to A-and P-sites (Qin et al.,2006).Despite a great number of recent biochemical studies,little remains known as to the precise physiological role of EF4 in vivo.In E.coli,80％ of EF4 molecules are associated with the cell membrane under optimal conditions,but when cells grow at pH 6 in media containing 100 mmol/L MgCl2,this percentage is reduced to less than 20％ (Pech et al.,2011).Furthermore,it was revealed that overexpression of EF4 in E.coli severely affected cell growth (Qin et al.,2006).However,EF4 knockout (△EF4,KO) cells exhibited no apparent phenotype in rich LB medium (Shoji et al.,2010).Under certain stresses,such as low temperature or high ionic stress,△EF4 cells were out-competed by wildtype (WT) cells within a short period of time (Shoji et al.,2010;Pech et al.,2011).