A STRUCTURAL MODEL FOR THE ESCHERICHIA-COLI DNAB HELICASE BASED ON ELECTRON-MICROSCOPY DATA

The DnaB protein is the major replicative DNA helicase in Escherichia coli. It hydrolyzes ATP to promote its translocation in the 5' to 3' d irection on single-stranded DNA templates, facilitating the separation of strands of duplex DNA in its path. This places it on the lagging s trands at replication forks during chromosomal DNA replication. Electr on microscopic images of negatively stained DnaB protein have been stu died and processed to produce a three-dimensional reconstruction of th e protein oligomer at 2.7 nm resolution. While it is known that the na tive protein is a complex of six identical 52-kDa subunits, the specim en shows threefold rather than sixfold symmetry, with three outer stai n-excluding regions surrounding another six, more massive, lobules. Th ere is a channel through the particle that appears fully open on both sides. Based on these results, a structural model for the oligomer is presented, and functional implications are considered. (C) 1995 Academ ic Press, Inc.