Organization and sequence of the SalI restriction-modification system

The organization and nucleotide (nt) sequences were determined for the genes encoding the Sal I restriction and modification (R-M) system (recognition sequence 5'-GTCGAC-3') from Streptomyces albus G. The system comprises two genes, salIR , coding for the restriction endonuclease (ENase, R · Sal I; probably 315 amino acids (aa), a predicted M r of 35 305; product, G'TCGAC) and salIM , coding for the methyltransferase (MTase, M Gb Sal I; probably 587 aa, a predicted M r of 64943; product, GTCGm 6 AC). The genes are adjacent, they have the same orientation, and they occur in the order salIR then salM . R · Sal I contains a putative magnesium-binding motif similar to those at the active sites of R · Eco RI and R · Eco RV, but otherwise it bears little aa sequence similarity to other ENases. M · Sal I is a member of the m 6 Aγ class of MTases. In aa sequence it resembles M · Acc I, another m 6 Aγ-MTase whose recognition sequence includes the Sal I recognition sequence as a subset.