Lactococcus lactis AbiD1 abortive infection efficiency is drastically increased by a phage protein.

Sensitivity of phage bIL66 to the AbiD1 Lactococcus lactis abortive infection mechanism was previously shown to be determined by the phage middle-time-expressed operon composed of four orfs. Using spontaneous bIL66 mutants resistant to AbiD1, we established that this sensitivity is determined by the orf1 encoded protein. Overproduction of Orf1 in trans in AbiD1(+) cells was shown to increase AbiD1 efficiency on both wild-type phage bIL66 and mutants resistant to AbiD1. Such an increase was not observed following overproduction of mutant Orf1. We propose that wild-type, but not a mutant Orf1, activates AbiD1 expression or activity.