Published online by Cambridge University Press: 14 April 2009
A complete genetic map has been established for the P22 clear plaque forming mutations cir4-l, cir5-l and cir6-l. These are located within or closely linked to the immI region of P22 and represent a new class of clear plaque forming mutants located outside and rather distant from the immC region. They were mapped with respect to the markers mnt, vy and ant of the immI region and to genes 16 and 9 which span it. The three cir mutations complement each other and – with one possible exception – the cl, c2 and c3 mutations of the immC region. P22 cir6-l – like P22 cir5-l (Harvey et al. 1981) – is suppressed by the ant−am19 allele, whereas P22 cir4-l is not. The results are discussed in terms of the regulation of early ant expression.