IN-VIVO SULFHYDRYL MODIFICATION OF THE LIGAND-BINDING SITE OF TSR, THE ESCHERICHIA-COLI SERINE CHEMORECEPTOR

The Escherichia coli chemoreceptor Tsr mediates an attractant response to serine. We substituted Cys for Thr-156, one of the residues involv ed in serine sensing. The mutant receptor Tsr-T156C retained serine- a nd repellent sensing abilities. However, it lost serine-sensing abilit y when it was treated in vivo with sulfhydryl-modifying reagents such as N-ethylmaleimide (NEM). Serine protected Tsr-T156C from these reage nts, We showed that [H-3]NEM bound to Tsr-T156C and that binding decre ased in the presence of serine. By pretreating cells with serine and c old NEM, Tsr-T156C was selectively labeled with radioactive NEM. These results are consistent with the location of Thr-156 in the serine-bin ding site. Chemical modification of the Tsr ligand-binding site provid es a basis for simple purification and should assist further in vivo a nd in vitro investigations of this chemoreceptor protein.