Vol. 273, Issue 3, 1329-1333, January 16, 1998

Biophysical Characterization of the TraY Protein of Escherichia coli F Factor

From the Department of Biology, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139

The TraY protein is required for efficient bacterial conjugation by Escherichia coli F factor. TraY has two functional roles: participating in the "relaxosome," a protein-DNA complex that nicks one strand of the F factor plasmid, and up-regulating transcription from the traYI promoter. The traY gene was cloned, and the TraY protein was expressed, purified, and characterized. TraY has a mixed -helix and -sheet secondary structure as judged by its circular dichroism spectrum, is monomeric, and undergoes reversible urea denaturation with G_u = 6 kcal/mol at 25 °C. The kinetics of protein unfolding and refolding, as measured by changes in fluorescence, are complex, suggesting the presence of intermediates or of heterogeneity in the folding reaction. TraY has been classified as a member of the ribbon-helix-helix family of transcription factors but is unusual in appearing to have tandem repeats of the motif in the same polypeptide chain. The data presented here show that folding and assembly of the functional ()₂ unit occurs as an intramolecular reaction and not by cross-folding between different polypeptide chains.