CHARACTERIZATION AND PRIMARY STRUCTURE OF A BASE NONSPECIFIC AND ACIDRIBONUCLEASE FROM DICTYOSTELIUM-DISCOIDEUM

A base non-specific and acid RNase was isolated from cellular slime mo ld (Dictyostelium discoideum) cells in a homogeneous state (about 2.4 kDa) by SDS-polyacrylamide gel electrophoresis. The RNase (RNase DdI) has a pH optimum of 5.0. The amino acid sequence of RNase DdI was dete rmined by a combination of protein chemistry, a search of Data base, D icty cDB and further sequence analysis of cDNA from the same bank. RNa se DdI consists of 198 amino acid residues, and about 13.3, 0.9, 1.2, 3.3, and 1.0 residues of mannose, xylose, glucose, GlcNAc, and GalNAc, respectively. RNase DdI has two characteristic conserved segments of the RNase T2 family, and thus belongs to the RNase T2 family. Consider ing the fact that most of the RNase activity of D. discoideum is prese nt in the lysosomal fraction [Wiener and Ashworth (1970) Biochem. J. 1 18, 505-512], it was concluded that the lysosomal RNase in D. discoide um is a member of the RNase T2 family. The amino acid sequence of RNas e DdI is highly homologous with that of Physarum polycephalum RNase (R Nase Phyb), and its amino acid sequence seems to be similar to those o f plant/animal type RNases, rather than fungal RNases. The location of RNase DdI in the phylogenetic tree of the RNase T2 family was estimat ed.