| Abstract: | In addition to 20 previously mapped restriction sites in the DNA of phi C31, we have determined eight sites for SphI, four for EcoRV, and two for SstII; there are none for BglII or SstI. Nine sites were in a 12-kb segment of DNA containing no previously mapped sites. Deletions causing clear-plaque morphology were located in this part of the DNA, in a 3-kb interval between an EcoRV and an SphI site at the centre of the DNA molecule. One of the deletions (delta C3) was obtained in a previously described phi C31c+::vph (viomycin phosphotransferase) derivative containing two PstI sites separated by 3.9-kb of inessential DNA. After in vitro PstI treatment, plaque-forming phages lacking the 3.9-kb fragment were obtained from the c+ phage but not from its delta C3 derivative. Thus a 36.2-kb genome, but not one of 34.4 kb, was able to give infectious virions. PstI-generated DNA fragments of up to 8 kb can be inserted in vitro into the delta C3 derivative with retention of the vph selective marker. With the insertion of a 6.03-kb PstI fragment of plasmid SCP2, the latter phage became a potential vector (with loss of vph) for BamHI-generated DNA fragments of up to 9 kb. In the course of this work, several ClaI sites in phi C31::pBR322 bifunctional replicons were shown to be lost when the DNA was propagated in a dam+ Escherichia coli strain. This will allow the use of such replicons for the cloning of ClaI-generated DNA fragments of up to 6.7 kb. |
