A physical gene map of the late region of the P22 chromosome has been constructed by genetic analysis of restriction enzyme fragments of P22 DNA cloned in a plasmid vector. Cleavage sites for restriction endonucleases SalI, SstI, SmaI, Xhoi, and BglI were mapped on P22 DNA to provide physical reference points in addition to the EcoRI, HindIII, and BamHI cleavage sites previously mapped. Restriction enzymes KpnI, BglII, and Xbal were found to have no cleavage sites on P22 DNA. Fragments of P22 DNA produced by cleavage with EcoRI, BamHI, or EcoRI plus BamHI were cloned in Escherichia coli using the plasmid vector pBR322, and the resulting recombinant plasmids were introduced into Salmonella typhimurium. The genes present on a cloned fragment were identified by the ability of the hybrid plasmid to complement or recombine with P22 amber mutations in known genes when mutant phage were used to infect S. typhimurium strains carrying the recombinant plasmids. These experiments place all phage genes required for P22 head morphogenesis except gene 3 on the physical map between coordinates 0.000 and 0.318. The coding capacity of this interval is in close agreement with the molecular weights of the proteins assigned to it. The single gene for the P22 base plate protein is placed between coordinates 0.376 and 0.420 on the physical map. These results also show that distances on the recombination frequency map are significantly distorted relative to the physical gene map of the late region. The recombination frequency map is expanded in the region of the physical gene map where terminally redundant ends of the circularly permuted mature chromosomes fall.
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