THE SPECIFICITY OF STYSKI, A TYPE-I RESTRICTION ENZYME, IMPLIES A STRUCTURE WITH ROTATIONAL SYMMETRY

The type I restriction and modification (R-M) enzyme from Salmonella e nterica serovar kaduna (StySKI) recognises the DNA sequence 5'-CGAT(N) (7)GTTA, an unusual target for a type I R-M system in that it comprise s two tetranucleotide components. The amino target recognition domain (TRD) of StySKI recognises 5'-CGAT and shows 36% amino acid identity w ith the carboxy TRD of EcoR124I which recognises the complementary, bu t degenerate, sequence 5'-RTCG, Current models predict that the amino and carboxy TRDs of the specificity subunit are in inverted orientatio ns within a structure with 2-fold rotational symmetry, The complementa ry target sequences recognised by the amino TRD of StySKI and the carb oxy TRD of EcoR1241 are consistent with the predicted inverted positio ns of the TRDs. Amino TRDs of similar amino acid sequence have been sh own to recognise the same nucleotide sequence. The similarity reported here, the first example of one between amino and carboxy TRDs, while consistent with a conserved mechanism of target recognition, offers ad ditional flexibility in the evolution of sequence specificity by incre asing the potential diversity of DNA targets for a given number of TRD s, StySKI identifies the first member of the IB family in Salmonella s pecies.