Cloning and Characterization of the Lactococcal Plasmid-Encoded Type II Restriction/Modification System, Lla DII

Abstract

ABSTRACT The Lla DII restriction/modification (R/M) system was found on the naturally occurring 8.9-kb plasmid pHW393 in Lactococcus lactis subsp. cremoris W39. A 2.4-kb Pst I- Eco RI fragment inserted into the Escherichia coli-L. lactis shuttle vector pCI3340 conferred to L. lactis LM2301 and L. lactis SMQ86 resistance against representatives of the three most common lactococcal phage species: 936, P335, and c2. The Lla DII endonuclease was partially purified and found to recognize and cleave the sequence 5′-GC↓NGC-3′, where the arrow indicates the cleavage site. It is thus an isoschizomer of the commercially available restriction endonuclease Fnu 4HI. Sequencing of the 2.4-kb Pst I- Eco RI fragment revealed two open reading frames arranged tandemly and separated by a 105-bp intergenic region. The endonuclease gene of 543 bp preceded the methylase gene of 954 bp. The deduced amino acid sequence of the Lla DII R/M system showed high homology to that of its only sequenced isoschizomer, Bsp 6I from Bacillus sp. strain RFL6, with 41% identity between the endonucleases and 60% identity between the methylases. The genetic organizations of the Lla DII and Bsp 6I R/M systems are identical. Both methylases have two recognition sites (5′-GCGGC-3′ and 5′-GCCGC-3′) forming a putative stem-loop structure spanning part of the presumed −35 sequence and part of the intervening region between the −35 and −10 sequences. Alignment of the Lla DII and Bsp 6I methylases with other m 5 C methylases showed that the protein primary structures possessed the same organization.