Mapping of deletions in a non-essential region of the phage T4 genome

Abstract

Many T4 r II deletions extend past the right-hand boundary of the r IIB cistron into a region of the genome which appears dispensible for growth on Escherichia coli B or K12 ( λ ). We have isolated RNA homologous to the DNA segment which is missing from a deletion mutant thought to extend farthest into the dispensible (D) region; this deletion-specific RNA was then hybridized with DNA samples of different deletion mutants suspected to extend some distance into region D. Since the percentage of RNA hybridized with DNA of a given deletion mutant should be inversely proportional to the length of the missing DNA segment, the hybridization data have yielded a crude map of the right-hand boundaries of r II deletions in region D. Independent isolates of nitrous acid-induced deletions, which were found to start at the same site in the r IIA cistron, were found to end mostly near the same site in the D region. We have interpreted this finding to imply an involvement of two partially homologous DNA regions in the formation of large deletions.