Ribonucleases of diverse specificities in rabbit brain nuclei

A salt extract of rabbit brain nuclei contains three endoribonucleases, designated RNases Y, A and R, which produce acid-soluble products when incubated at near-neutral pH in the absence of metal ions. RNases Y and A yield products with the monoesterified phosphate at the 3' position, through 2',3'-(cyclic)phosphate intermediates. Oligonucleotides terminating with a 2',3'-(cyclic)phosphate are the end-products of the action of RNase R. Double-stranded substrates are highly resistant to the action of all enzymes. On the basis of limited hydrolysis of end-labelled 5S RNA, the three enzymes differ in their preference for the susceptible phosphodiester bond. Thus, RNase Y hydrolyses preferentially the YpN bond, RNase A the ApN bond and RNase R the RpU bond where R is guanosine in most cases. The advantages and disadvantages of using homopolyribonucleotides and dephosphorylated dinucleotides and trinucleotides in determining various aspects of the specificity of RNases are discussed.