The fungal ribonucleases T2 from Aspergillus oryzae, M from Aspergillus saitoi and Rh from Rhizopeus niveus are structurally and functionally related 30 Kd glycoproteins [1] that cleave the 3'-5' internucleotide linkage of RNA via a nucleotide 2',3'-cyclic phosphate intermediates (EC 3.1.27.1).

A number of other RNAses have been found to be evolutionary related to these fungal enzymes:

• Self-incompatibility [2] in flowering plants is often controlled by a single gene (S-gene) that has several alleles. This gene prevents fertilization by self-pollen or by pollen bearing either of the two S- alleles expressed in the style. The self-incompatibility glycoprotein from several higher plants of the solanaceae family has been shown [2,3] to be a ribonuclease.
• Phosphate-starvation induced RNAses LE and LX from tomato [4]. These two enzymes are probably involved in a phosphate-starvation rescue system.
• Escherichia coli periplasmic RNAse I (EC 3.1.27.6) (gene rna) [5].
• Aeromonas hydrophila periplasmic RNAse.
• Haemophilus influenzae hypothetical protein HI0526.

Two histidines residues have been shown [6,7] to be involved in the catalytic mechanism of RNase T2 and Rh. These residues and the region around them are highly conserved in all the sequence described above. We have developed two signature patterns, one for each of the two active-site histidines. The second pattern also contains a cysteine which is known to be involved in a disulfide bond.