DNA-SEQUENCE DETERMINANTS OF LEXA-INDUCED DNA BENDING

The LexA repressor from Escherichia coli induces DNA bending upon inte raction with the two overlapping operators which regulate the transcri ption of the colicin A encoding gene caa. Both caa operators harbor T- tracts adjacent to their recognition motifs. These tracts have been su ggested to be especially favorable for the promotion of LexA-induced D NA bending. Here we show that this is indeed the case, since disruptio n of the TTTT-tract adjacent to operator O1 by the replacement of the two central thymine bases by AA, GA or CG markedly reduces LexA-induce d DNA bending. Simple A . T-richness in this position is thus not suff icient to promote full LexA-induced bending, albeit a TAAT sequence is always more efficient to promote bending than those sequences contain ing one or two C/G base pairs.