An endonuclease that primarily cleaves single-stranded substrates, but can also act on double-stranded DNA or RNA. It hydrolyzes 5′ phosphodiester bonds, generating 3′-mononucleotides and oligonucleotides. Enzyme activity depends on Ca2+ and can be inhibited by EDTA. Shows high specificity for AT-rich regions compared to GC-rich DNA. Widely used in ribonuclease selection studies and ribosome analysis.