Natural history of a viral cohesive end site: cosN of the λ-like phages

The base pairs of cosN, the site where the 12 base-long cohesive ends are generated in lambda-like phages, show partial-two fold rotational symmetry. In a bioinformatic survey, we found that the cosN changes in 12 natural cosN variants are restricted to bp 6-to-12 of the cohesive end sequence. In contrast, bp 1-5 of the cohesive end sequence are strictly conserved (13/13), as are the two bp flanking the left nicking site (bp -2 and 1). The bp flanking the right nick site (bp 13 and 14) are conserved in 12 of 13 variants. Five cosN variants differing by as many as five bp were used to replace lambda's cosN. No significant effects of the cosN changes on lambda's virus yield were found. In sum, bp -2 to 5 are critical cosN function, and bp 6-12 of the cohesive end sequence are not critical for terminase recognition or virus fitness.