Figure 1.

Construction of λ ZAP III and cloning of dinucleotide repeats. λ Zap III was constructed by cloning the entire lacZ gene into λ ZAP II. (AC)₃₉ and (TG)₃₃ repeats purified from pBS II SK⁺ (AC)₃₉ and pBS II SK⁺ (TG)₃₃ were introduced into λ ZAP III by using two restriction enzymes, NotI and EcoRI, to ensure the directionality of the insertion. Integration of λ ZAP III at the λ attachment site directs the insertion of the (AC) or (TG) sequences on the leading or lagging strand of the E. coli chromosome. The position of the two kinds of repeats with respect to the direction of replication was verified by sequencing, by using primers specific for the leading strand. See Materials and Methods for more details.