1. Institut für Biochemie und Molekularbiologie, Hermann-Herder-Strasse 7, 79104 D-Freiburg, Germany

Correspondence to: Bernd Bukau¹ Correspondence and requests for materials should be addressed to B.B. (e-mail: Email: bukau@sun2.ruf.uni-freiburg.de).

The role of molecular chaperones in assisting the folding of newly synthesized proteins in the cytosol is poorly understood. In Escherichia coli, GroEL assists folding of only a minority of proteins¹ and the Hsp70 homologue DnaK is not essential for protein folding or cell viability at intermediate growth temperatures². The major protein associated with nascent polypeptides is ribosome-bound trigger factor³,⁴, which displays chaperone and prolyl isomerase activities in vitro³,⁵,⁶. Here we show that tig::kan mutants lacking trigger factor have no defects in growth or protein folding. However, combined tig::kan and dnaK mutations cause synthetic lethality. Depletion of DnaK in the tig::kan mutant results in massive aggregation of cytosolic proteins. In tig::kan cells, an increased amount of newly synthesized proteins associated transiently with DnaK. These findings show in vivo activity for a ribosome-associated chaperone, trigger factor, in general protein folding, and functional cooperation of this protein with a cytosolic Hsp70. Trigger factor and DnaK cooperate to promote proper folding of a variety of E. coli proteins, but neither is essential for folding and viability at intermediate growth temperatures.