The LlaGI restriction and modification system of Lactococcus lactis W10 consists of only one single polypeptide

The naturally occurring 12.1-kb plasmid, pEW104, in Lactococcus lactis ssp. cremoris W10 was found to confer decreased bacteriophage sensitivity to its host. Plasmid pEW104 encodes a non-classic restriction and modification (R/M) system, named LlaGI, consisting of only one single polypeptide. Analysis of the amino acid sequence revealed the presence of a catalytic motif and seven helicase-like motifs (DEAD-box motifs) characteristic of type I and III endonucleases, followed by four conserved methylase motifs characteristic of adenine-methylases. A comparison between LlaGI and the very similar R/M system, LlaBIII, suggests that the C-terminal region of LlaGI, apparently containing no known motifs, could possibly specify target DNA recognition. Conceivably, the LlaGI gene is included in the operon of the plasmid replication machinery. Finally, it is proposed that LlaGI represents a variant of the type I R/M systems.