No abstract

The DNA of bacteriophage BF23 possesses two structural features, localized single-chain interruptions and a large terminal repetition, previously described for T5, a closely related virus. As is the case for T5, single-chain interruptions occur with variable frequencies at a small number of fixed sites within one strand of the double-stranded BF23 genome. The sites where interruptions occur with the highest frequencies were mapped by an electrophoretic analysis of the singlestranded fragments produced by denaturation of BF23 DNA. The positions of these fragments were determined by degrading BF23 DNA to various extents with A exonuclease and observing the relative order with which they were (i) degraded or (ii) released intact from the undenatured duplex. The exact locations of the interruptions were determined from analysis of analogous duplex fragments produced by degrading exonuclease Ill-treated BF23 DNA with a single-strandspecific endonuclease. BF23 has five principal sites (located at 7.9, 18.7, 32.4, 65.8, and 99.6% from the left end of the DNA) where interruptions occur in most molecules. The principal interruptions in T5 DNA occur at similar positions. The locations of eight secondary interruptions in BF23 DNA were also determined. In general, BF23 DNA has fewer secondary interruptions than T5 DNA, although there is at least one location where an interruption occurs with a greater frequency in BF23. The presence bf a terminal repetition in BF23 DNA was demonstrated by annealing ligase-repaired molecules that had been partially digested with A exonuclease. If the complementary sequences at both ends of the DNA were exposed by exonuclease treatment, the duplex segment that resulted from annealing could be released by digestion with a single-strand-specific endonuclease. This segment was analyzed by agarose gel electrophoresis and found to represent 8.4% of BF23 DNA. nated BF23st(+), were obtained from J. Mc-Corquodale and G. Hayward. Two heat-stable deletion mutants, BF23st(102) and BF23st(104), were isolated 777

Cleavage maps of bacteriophage BF23 DNA have been constructed for the restriction endonucleases Sall (3 fragments), BamHI (5 fragments), EcoRI (8 fragments), BailI (13 fragments), and HpaI (49 fragments, 32 of which have been ordered). The maps were determined by (i) analysis of deletion mutants, (ii) digestion with two endonucleases, (iii) digestion of isolated fragments with a second enzyme, (iv) analysis of partial digests, and (v) digestion after treatment with A exonuclease.