Presence of random single-strand gaps in mycobacteriophage I3 DNA

The genomic double-stranded DNA of mycobacteriophage I3, when denatured with alkali, heat, formamide or dimethylsulfoxide, breaks down to heterogeneous-sized single-strand (ss) fragments smaller than the expected intact unit genome length suggesting the presence of random ss interruptions on both the strands. The occurrence of the interruptions at random is also demonstrated by two-dimensional gel electrophoresis of the restriction fragments of I3 DNA. These interruptions have no adverse effect on the phage infectivity or DNA transfectivity. Studies with nuclease BAL 31 and end-labeling analysis confirm the presence of random interruptions. Detailed analysis using T4 DNA ligase, nuclease S1 and DNA polymerase I Klenow fragment revealed that the interruptions are in the form of small gaps rather than single phosphodiester bond breaks. The average length of the gap is about 10 nucleotides long and there are 13 to 14 such gaps per DNA molecule.