Figure 1.

MMR regulation by SpyCIM1 in strain SF370. Prophage-like chromosomal island SpyCIM1 integration separates mutL, lmrP, ruvA, and tag from the promoter upstream of mutS, preventing expression of these genes and leading to a mutator phenotype in strain SF370 (Scott et al., 2008). mutS is constitutively expressed from mRNA_A, which is truncated by the presence of the SpyCI. When cells enter early logarithmic growth phase, approximately at the time of initiation of DNA replication, the SpyCI excises from the host chromosome and restores transcription of the polycistronic message from mutS to tag (mRNA_B). The excised phage circularizes and replicates in the host cell as an episome. As cell densities increase, the phage genome integrates into attB located at the 5′ end of mutL, returning the cell to the mutator phenotype. Large, unfilled arrows indicate transcriptionally inactive genes of the MMR operon (mutS, mutL, lmrP, ruvA, and tag), while transcriptionally active ones are large arrows filled in black. Small, filled arrows indicate the SpyCI ORFs; except for int (Scott et al., 2008), the transcriptional patterns of the SpyCI genes are unknown at this time.