CLONING AND SEQUENCING OF LLAII RESTRICTION-MODIFICATION GENES FROM LACTOCOCCUS-LACTIS AND RELATEDNESS OF THIS SYSTEM TO THE STREPTOCOCCUS-PNEUMONIAE DPNII SYSTEM

The natural 7.8-kb plasmid !pSRQ700 was isolated front Lactococcus lac tis subsp. cremoris DCH-4. It encodes a restriction/modification syste m named LlaII. When introduced into a phage-sensitive L. lactis strain , pSRQ700 confers strong phage resistance against the three most commo n lactococcal phage species,namely, 936, c2, acid P335. The LlaII endo nuclease was purified and found to cleave the palindromic sequence 5'- GATC-3'. It is an isoschizomer of Streptococcus pneumoniae DpnII. The plasmid pSRQ700 was mapped, and the genetic organization of LlaII was localized. Cloning and sequencing of the entire LlaII system allowed t he identification of three open reading frames. The three genes (llaII B, llaIIB, and llaIIC) overlapped and are under one putative promoter, A putative terminator was found at the end of llaIIC. The genes llaII A and llaIIB coded for m(6)A methyltransferases, and llaIIC coded for: an endonuclease. The LlalI system shares strong genetic similarities with the DpnII system. The deduced amino acid sequence of M.LlaIIA was 75% identical with that of M.DpnII, whereas M.LlaIIB was 88% identica l with M.DpnA. However, R.LlaIII shared only 31% identity with R.DpnII .