Crossover sites cix for inversion of the invertible DNA segment C on the bacteriophage P7 genome.

The bacteriophage P7 genome contains an invertible DNA segment called C which determines its host range. P7 C(+) phages produce plaques on Escherichia coli K12. The C segment consists of a 3-kb unique sequence and 0.62-kb inverted repeats of which one carries an internal 0.2-kb deletion. This deletion has been mapped within the right inverted repeat in the C(+) orientation. The crossover sites cix for inversion of the C segment do not map at the inside boundaries of the inverted repeats, as had been proposed. They are localized at the external ends of these repeats. Thus organization of the C segment in phage P7 is analogous to that in the related phage P1.