Group Y incompatibility and copy control of P1 prophage

Abstract

We have identified a restriction fragment ( EcoRI-5) of bacteriophage P1 that, when cloned in a λ prophage, expresses incompatibility characteristic of the unit copy P1 plasmid prophage. Lysogens of λ-P1 chimeras in which the P1 fragment is EcoRI-5 fail to maintain P1 or P7 plasmids. In order to study the nature of this incompatibility, we isolated P1 mutants that overcome it. These mutants exhibit an elevated copy number. We provide evidence that the increased copy number results from a defect in a repressor of replication that can be furnished in trans by a chromosomally integrated P1, but not by EcoRI-5 itself. We, therefore, suggest that the incompatibility exerted by EcoRI-5 is not attributable to the represser of replication involved in the above copy control defect. Instead, it could be attributed to the presence of a DNA site required for proper plasmid partition at cell division. The elevated copy number of the P1 mutants would then enable them to compete favorably with the single copy of the cloned EcoRI fragment for a cellular component of the partition apparatus. Thus, incompatibility could be overcome.