Nucleotide sequence of bacteriophage λ DNA

The nucleotide sequence of the DNA of bacteriophage λ has been determined using the dideoxy chain termination method in conjunction with random cloning in M13 vectors. Various methods were studied for sequencing specific regions to complete the sequence, but all were much slower than the random approach. The DNA in its circular form contains 48,502 base-pairs. Open reading frames were identified and, where possible, ascribed to genes by comparing with the previously determined genetic map. The reading frames for 46 genes were clearly identified, though in about 20 the position of the protein initiation site could not be rigorously established. Probable positions for the kil , cIII and lom genes are suggested but remain uncertain. There are about 20 other unidentified reading frames that may code for proteins. The genome is fairly compact with comparatively little non-coding DNA. In many cases the translation terminators and initiators overlap, particularly in the sequence A-T-G-A where the TGA terminates one gene and the ATG initiates the next. Such structures seem to be characterized by a purine-rich sequence, rather than by a specific “Shine and Dalgarno” sequence, before the initiator. In the whole of the left arm the codon CTA, which is normally read by a minor leucine tRNA, is absent. The distribution of other rare codons in the genes of the left arm suggests that they may have a controlling function on the relative amounts of the proteins produced.