Effects of DNA heterologies on bacteriophage lambda packaging.

We have examined the impact of DNA heterologies on the packaging of lambda DNA in vitro. Heterology-containing DNA molecules were constructed by denaturing and reannealing a mixture of DNA from cI+ phage and DNA front phage carrying small insertion or deletion mutations in the cI gene. We found that molecules with heterologies of up to 19 base pairs (bp) can be packaged as viable heterozygous phage with approximately the same efficiency as molecules with a base pair mismatch. In contrast, with a heterology of 26-bp heterozygous plaque formers are rare. In principle, the absence of cI heterozygotes among packaged phage may be due either to a failure to encapsulate the DNA or a failure to inject the packaged DNA on infection. Southern blot analysis of DNA isolated from packaged phage indicates that DNA harboring a 26-bp heterology is almost completely absent in packaged phage. Thus, an upper limit has been established for the size of heterology that can be accommodated by the packaging apparatus The size of the connector portal could be the basis for this limit.