DNA sequence determinants of LexA-induced DNA bending

The LexA repressor from Escherichia coli induces DNA bending upon interaction with the two overlapping operators which regulate the transcription of the colicin A encoding gene caa. Both caa operators harbor T-tracts adjacent to their recognition motifs. These tracts have been suggested to be especially favorable for the promotion of LexA-induced DNA bending. Here we show that this is indeed the case, since disruption 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-induced DNA bending. Simple A.T-richness in this position is thus not sufficient to promote full LexA-induced bending, albeit a TAAT sequence is always more efficient to promote bending than those sequences containing one or two C/G base pairs.