Polypeptide flux through bacterial Hsp70: DnaK cooperates with trigger factor in chaperoning nascent chains

A role for DnaK, the major E. coli Hsp70, in chaperoning de novo protein fo lding has remained elusive. Here we show that under nonstress conditions Dn aK transiently associates with a wide variety of nascent and newly synthesi zed polypeptides, with a preference for chains larger than 30 kDa. Deletion of the nonessential gene encoding trigger factor, a ribosome-associated ch aperone, results in a doubling of the fraction of nascent polypeptides inte racting with DnaK. Combined deletion of the trigger factor and DnaK genes i s lethal under normal growth conditions. These findings indicate important, partially overlapping functions of DnaK and trigger factor in de novo prot ein folding and explain why the loss of either chaperone can be tolerated b y E. coli.