CA RUNS INCREASE DNA FLEXIBILITY IN THE COMPLEX OF LAMBDA-CRO PROTEINWITH THE OR3 SITE

The alternating pyrimidine-purine elements CA, CAC, and CACA are aniso tropically flexible, as deduced from gel circularization assays on poi nt mutations and single-base mismatches in the O(R)3 site of lambda ph age alone and in the specific complex with the Cro protein. These sequ ences evidently promote DNA bending in the specific binding region of the complex and may also facilitate overwinding in the central nonbind ing region. Effects for CACA are exceptionally large and suggest that an alternative DNA structure may occur in this element.