A calcium dependent endoribonuclease from Trypanosoma congolense cytoplasm

A major endoribonuclease whose activity depends on the presence of calcium ions was isolated from the cytoplasm of Trypanosoma congolense. Like the Ca2+-dependent endoribonuclease of T. brucei cytoplasm, this enzyme degraded poly(A) preferentially but also degraded to a lesser extent poly(U), tRNA and rRNA to give 5'-phosphoribosyl oligonucleotides. Neither enzyme hydrolyzes poly(G) or DNA. Both ribonucleases are inhibited by polyanions and heavy metal ions, and are heat-labile; N-ethylmaleimide has no effect on either activity. However, unlike the Ca2+-RNase of T. brucei cytoplasm, this endoribonuclease is also stimulated by Mg2+, acts maximally at around pH 8.9, and is virtually inactivate towards poly(C). It is inhibited by iodoacetamide but unaffected by spermine. Its molecular weight is about 42 000 as determined by gel filtration.