Catalyses the two-stage endonucleolytic cleavage of nucleoside 3'-phosphates and 3'-phosphooligonucleotides ending in A-P or G-P with 2',3'-cyclic phosphate intermediates. The enzyme contains an isoaspartate residue.
Defined by 4 residues: HIS:A-41GLU:A-62ARG:A-85HIS:A-101