Bacteriophage Mu DNA replication is stimulated by non-essential early functions

The replication of a spontaneous Kil- mutant of bacteriophage Mu has been investigated. The Kil- mutation (Mucts62-13/4), which was introduced into a defective prophage, is pleiotrophic, leading to the loss of also the Gam, Cim and Sot functions. The mutation is caused by an insertion with the characteristics of IS1, located just outside the B gene. Mucts62-1 3/4 phages form extremely small plaques on wildtype indicator strains. The replication of the insertion mutant as compared to Mucts62 is strongly reduced. Normal replication could be restored by relieving the polarity of the insertion or by complementation with defective prophages which express all early functions. Apparently, early genes other than A and B are involved in normal Mu DNA replication.