Cloning and sequence analysis of the Stsl restriction-modification gene: presence of homology to Fokl restriction-modification enzymes

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Abstract
Stsl endonuclease (R.Stsl), a type Us restriction endonuclease found in Streptococcus sanguls 54, recognizes the same sequence as Fokl but cleaves at different positions. A DNA fragment that carried the genes for R.Stsl and Stsl methylase (M.Stsl) was cloned from the chromosomal DNA of S.sanguls 54, and its nucleotide sequence was analyzed. The endonuclease gene was 1,806 bp long, corresponding to a protein of 602 amino acid residues (Mr = 68,388), and the methylase gene was 1,959 bp long, corresponding to a protein of 653 amino acid residues (Mr = 76,064). The assignment of the endonuclease gene was confirmed by analysis of the N-terminal amino acid sequence. Genes for the two proteins were In a tail-totail orientation, separated by a 131-nucleotide intercistronic region. The predicted amino acid sequences between the Stsl system and the Fokl system showed a 49% identity between the methylases and a 30% identity between the endonucleases. The sequence comparison of M.Stsl with various methylases showed that the N-terminal half of M.Stsl matches M.Nlalll, and the C-terminal half matches adenine methylases that recognize GATC and GATATC.