Site of Alkylation of the Major Ribonuclease from Aspergillus saitoi with Iodoacetate

A base non-specific and adenyhc acid preferential ribonuclease from Aspergillus saitoi (RNase M) was modified by ["Qiodoacetic acid. RNase M was inactivated with concomitant incorporation of about 1 mol equivalent of carboxymethyl group. Carboxymethylated RNase M (CM RNase M) thus obtained was reduced and carboxymethylated (RCM CM RNase M). From tryptic and chymotryptic digests of RCM CM RNase M, two carboxymethylated histidine-containing peptides labeled with radioactivity were isolated. The amino acid sequences of these two peptides were determined to be Thr-Ile-His-Gly-Leu-Trp-Pro-Asp-Asn-Cys-Asp-Gly-Ser-Tyr and His-Gly-Thr-Cys-lle-Asn-Thr-lle-Asp-Pro-Ser-Cys-Tyr-Pro-Asp-Asp-Tyr-Ala. The distribution of the radioactivity on the former and latter peptides was 43 % and 57%, respectively. The results indicated that two histidine residues are involved in the active site of RNase M, and the modification of either one of the two histidine residues inactivates RNase M. The CD spectrum of carboxymethylated RNase M indicated that some tryp-tophan residue(s) with a CD band at 287 nm is in the proximity of the active sit