lac repressor changes conformation upon binding to poly[dA-T)]

Abstract

N-(iodoacetylaminoethyl)-1-naphthylamine-5-sulfonate reacts with Escherichia coli lac repressor to selectively label cysteine-140 with the fluorescent N-(acetylaminoethyl)-1-naphthylamine-5-sulfonate group. The fluorescence intensity of this label decreases by 20% when labeled repressor associates with poly[d(A-T)]. Fifteen base pairs of poly[d(A-T)] per repressor tetramer are required to complete this decrease. Stopped-flow experiments showed that the repressor undergoes at least 2 conformational changes as it binds to poly[d(A-T)], with half-lives of 5.0 .+-. 1.2 ms and 3.5 .+-. 1.0 s. Quite likely, these conformational changes serve to strengthen the interaction of repressor with DNA.