SEQUENCE OF THE SALMONELLA-TYPHIMURIUM STYLT1 RESTRICTION-MODIFICATION GENES - HOMOLOGIES WITH ECOP1 AND ECOP15 TYPE-III R-M SYSTEMS AND PRESENCE OF HELICASE DOMAINS

The StyLT1 restriction-modification (R-M) system of Salmonella typhimu rium has recently been suggested to belong to the type-III R-M systems [De Backer and Colson, Gene 97 (1991) 103-107]. The nucleotide sequen ces of StyLT1 mod and res have been determined. Two closely adjacent o pen reading frames were found 12 bp apart with coding capacities of 65 1 (Mod) and 982 (Res) amino acids (aa), respectively. The genes, lying in the same direction of transcription in the mod-res order, are tran scribed as distinct units. The deduced aa sequences reveal homologies with known type-III enzymes from the Escherichia coli P1 prophage, E. coli P15 plasmid and Bacillus cereus chromosome. In addition, the StyL T1 restriction endonuclease (ENase), like other type-I and type-III EN ases, contains sequence motifs characteristic of superfamily-II helica ses, which may be involved in DNA unwinding at the cleavage site.