Expression, Regulation, and Mode of Action of the AbiG Abortive Infection System of Lactococcus lactis subsp. cremoris UC653

The abortive infection system AbiG is encoded by the lactococcal plasmid pCI750. The abiG locus (consisting of two genes, abiGi and abiGii) was examined by Northern blot analysis, revealing two transcripts of approximately 2.8 and 1.5 kb which were homologous to the two gene-specific probes. A transcriptional start site was mapped upstream of abiGi, and it appeared that the two genes were cotranscribed, resulting in the 2.8-kb transcript. The smaller transcript may be the result of independent transcription of abiGii within abiGi or of the presence of a weak terminator within abiGii. The locus was shown to be constitutively expressed. Evidence is presented for the possible existence of a second Abi mechanism on pCI750. Examination of phage sk1 RNA synthesis demonstrated that both the subcloned AbiG and, to a greater extent, pCI750 inhibited this process. pCI750 also severely inhibited synthesis of both early and late phage c2 transcripts, while the presence of the subclone resulted in a reduction in late transcript synthesis only.