ORGANIZATION AND SEQUENCE OF THE SALI RESTRICTION-MODIFICATION SYSTEM

The organization and nucleotide (nt) sequences were determined for the genes encoding the SalI restriction and modification (R-M) system (re cognition sequence 5'-GTCGAC-3') from streptomyces albus G. The system comprises two genes, salIR, coding for the restriction endonuclease ( ENase, R.SalI; probably 315 amino acids (aa), a predicted M(r) of 35 3 05; product, G'TCGAC) and sal1M, coding for the methyltransferase (MTa se, M.SalI; probably 587 aa, a predicted M(r) of 64943; product, GTCGm (6)AC). The genes are adjacent, they have the same orientation, and th ey occur in the order salIR then salIM. R.SalI contains a putative mag nesium-binding motif similar to those at the active sites of R.EcoRI a nd R.EcoRV, but otherwise it bears little aa sequence similarity to ot her ENases. M SalI is a member of the m(6)A gamma class of MTases. In aa sequence it resembles M.AccI, another m(6)A gamma-MTase whose recog nition sequence includes the SalI recognition sequence as a subset.