X-ray Crystal Structure of the Bacterial Conjugation Factor PsiB, a Negative Regulator of RecA

Vessela Petrova; Kenneth A. Satyshur; Nicholas P. George; Darrell McCaslin; Michael M. Cox; James L. Keck

doi:10.1074/jbc.M110.152298

X-ray Crystal Structure of the Bacterial Conjugation Factor PsiB, a Negative Regulator of RecA*

From the ^‡Program in Cellular and Molecular Biology and
Departments of ^§Biomolecular Chemistry and
^¶Biochemistry, University of Wisconsin, Madison, Wisconsin 53706-1532

1 To whom correspondence should be addressed: Dept. of Biomolecular Chemistry, University of Wisconsin School of Medicine and Public Health, 550 Medical Science Center, 1300 University Ave., Madison, WI 53706-1532. Tel.: 608-263-1815; Fax: 608-262-5253; E-mail: jlkeck{at}wisc.edu.

Abstract

During bacterial conjugation, genetic material from one cell is transferred to another as single-stranded DNA. The introduction of single-stranded DNA into the recipient cell would ordinarily trigger a potentially deleterious transcriptional response called SOS, which is initiated by RecA protein filaments formed on the DNA. During F plasmid conjugation, however, the SOS response is suppressed by PsiB, an F-plasmid-encoded protein that binds and sequesters free RecA to prevent filament formation. Among the many characterized RecA modulator proteins, PsiB is unique in using sequestration as an inhibitory mechanism. We describe the crystal structure of PsiB from the Escherichia coli F plasmid. The stucture of PsiB is surprisingly similar to CapZ, a eukaryotic actin filament capping protein. Structure-directed neutralization of electronegative surfaces on PsiB abrogates RecA inhibition whereas neutralization of an electropositive surface element enhances PsiB inhibition of RecA. Together, these studies provide a first molecular view of PsiB and highlight its use as a reagent in studies of RecA activity.

Footnotes

↵* This work was supported, in whole or in part, by National Institutes of Health Grants GM32335 (to M. M. C.) and GM068061 (to J. L. K.).
The atomic coordinates and structure factors (code 3NCT) have been deposited in the Protein Data Bank, Research Collaboratory for Structural Bioinformatics, Rutgers University, New Brunswick, NJ (http://www.rcsb.org/).