| Abstract: | We studied the ability of lambda, phi 80 and their hybrid lambda att80 to lysogenize homoimmune monolysogens and examined the prophage locations on the chromosome of the resulting polylysogens. We observed an effective integration of phi 80 and lambda att80, in contrast to lambda, into the host chromosome, exclusively, at the attachment sites that were not occupied by the resident prophage (nontandem). Besides, the lambda att80 (int+) prophage was observed to ensure effective nontandem integration of a homoimmune int mutant DNA. Hence, we inferred that the expression of the int gene in the phi 80 prophage is constitutive, cI-independent and results in nontandem integration of the homoimmune prophage. The validity of this inference has been supported experimentally: (i) the only lysogen that was found to contain a phi 80 tandem was highly unstable (spontaneous segregation of monolysogens occurred 6-7 times more frequently than with the lambda tandem); (ii) an int inactivating mutation stabilized the phi 80 tandem; as a result, the int mutant has the frequency of tandem integration as high as that of lambda, while no nontandem integration was observed. A hypothesis is proposed which accounts for the instability of the phi 80 tandems and explains the relation between this phenomenon and the prophage ability to integrate into secondary attachment sites in the presence of the primary (normal) one. |
