Sequence organization of the origins of DNA replication in lambdoid coliphages

We have determined the sequences of the oriregion DNA of several phage λ mutants and hybrids, which shed light on the mechanism of DNA replication in the lambdoid phages. These include the heterologous substitution hybrids λrep82:λ and λrep80:λ, a pseudorevertant of the ori−r93 mutant λr93 mutant λr93hot5, and the insertion mutant λpk35. The oriregions of the three lambdoid phages, λ, φ80 and 82, all have repeated sequences, termed iterons, and A·T-rich zones. We note that a similar arrangement of DNA is also found in several other prokaryotic origins of replication. λ and φ80 have four iterons, and 82 has five. The origin of λr93hot5 is unusual in that contains only three iterons, yet the phage grows normally. Analysis of this mutant indicates that the spacing of iterons is crucial to orifunction, whereas their number is not. This argues against the cloverleaf model for λ oristructure (Hobom et al., 1979). In λpk35 the drug resistance element Tn903is inserted into the “inceptor” (ice) site, proposed to be crucial for λ replication initiation (Hobom et al., 1979); yet this phage grows normally.