SEQUENTIAL FOLDING OF UMUC BY THE HSP70 AND HSP60 CHAPERONE COMPLEXESOF ESCHERICHIA-COLI

Replication-blocking lesions generate a signal in Escherichia coil tha t leads to the induction of the multigene SOS response. Among the SOS- induced genes are umuD and umuC, whose products are necessary for the increased mutation rate in induced bacteria. The mutations are likely to result from replication across the DNA lesion, and such a bypass ev ent has been reconstituted in vitro (Rajagopalan, M., Lu, C., Woodgate , R., O'Donnel, M., Goodman, M. F., Echols, H. (1992) Proc. Natl. Acad . Sci. U. S. A. 89, 10777-10781). In this work, we show that the chape rone proteins promote the proper folding of UmuC protein in vitro. We treated purified and inactive UmuC with Hsp70 and Hsp60. After Hsp70 t reatment, the DNA binding activity of UmuC was recovered, but the abil ity to promote replication across DNA lesions was not. However, lesion bypass activity was recovered upon further treatment with Hsp60. The biological significance of such a folding pathway for UmuC protein is strengthened by in vivo evidence for a role of DnaK in UV-induced muta genesis.