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 (recognition 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 Mr of 35 305; product, G'TCGAC) and salIM, coding for the methyltransferase (MTase, M Gb SalI; probably 587 aa, a predicted Mr of 64943; product, GTCGm6AC). The genes are adjacent, they have the same orientation, and they occur in the order salIR then salM. R · SalI contains a putative magnesium-binding motif similar to those at the active sites of R · EcoRI and R · EcoRV, but otherwise it bears little aa sequence similarity to other ENases. M · SalI is a member of the m6Aγ class of MTases. In aa sequence it resembles M · AccI, another m6Aγ-MTase whose recognition sequence includes the SalI recognition sequence as a subset.