DNA recognition by a new family of type I restriction enzymes: a unique relationship between two different DNA specificities.

The DNA sequences recognized by the allelic type I restriction enzymes EcoR124 and EcoR124/3 were determined. EcoR124 recognizes 5'-GAA(N6)RTCG-3' and EcoR124/3 recognizes 5'-GAA(N7)RTCG-3'. These are typical of sequences recognized by type I recognition enzymes in that they consist of two specific domains separated by a non-specific spacer sequence. For these two enzymes, the specific sequences are identical but the length of the non-specific spacer is different. The specific domains of EcoR124/3 are thus 3.4 A further apart than those of EcoR124 and rotated with respect to each other through a further 36 degrees.