Nucleotide distribution and functional orientation in the deoxyribonucleic acid of phage phi 80

Abstract

The distribution of nucleotides in the deoxyribonucleic acid (DNA) of phi80 was determined by density analysis of molecular fragments of known length and origin. One half of the molecule includes a long, fairly homogeneous segment that contains 55% guanine plus cytosine (GC). DNA in the other half contains about 50% GC, except for a short stretch near the molecular end where the GC content is higher. Transcription of phi80 DNA was studied by ribonucleic acid-DNA hybridization tests with isolated molecular halves. At times early in the growth cycle, messenger is synthesized at a constant, relatively low rate and originates almost exclusively from DNA in the lower-GC half. At later times, messenger represents both halves and is synthesized at a greatly increased rate. The DNA of phi80 is closely analogous, physically and functionally, to lambda DNA. The similarity is most striking in the high-GC half, which is defined here as "left" since its molecular end contains a cohesive site homologous to that at lambda's left end. The left halves of the DNA of the two phages contain primarily late-functioning genes, and possess similar nucleotide distributions and some similar base sequences. The right halves of the two DNA molecules are less similar. Both contain the early genes, but they differ considerably in GC content (45% in lambda, 51% in phi80) and do not strongly cross-react in hybridization tests. The DNA of phi80 lacks the central 37%-GC segment found in lambda.