RNA 3'-terminal phosphate cyclase (EC 6.5.1.4) [1,2] catalyzes the conversion of 3'-phosphate to a 2',3'-cyclic phosphodiester at the end of RNA. The biological role of this enzyme is unknown but it is likely to function in some aspects of cellular RNA processing.

The reaction catalyzed by the enzyme occurs in three steps:

 1) adenylation of the enzyme by ATP;
 2) the  enzyme  acts  on  RNA-3'terminal  phosphate to produce RNA-3'terminal
    diphosphate adenylate;
 3) Release  of  AMP and cyclisation by a non catalytic nucleophilic attack by
    the adjacent 2'hydroxyl on the phosphorus in the diester linkage.

This enzyme, which has been characterized in human (where there seems to be at least three isozymes) and Escherichia coli (gene rtCA), seems to be taxonomically widespread. It is found in insects, plants, fungi (gene RTC1 in yeast) and in archeabacteria.

RNA cyclase is a protein of from 36 to 42 Kd. The best conserved region, which we use as a signature pattern, is a glycine-rich stretch of residues located in the central part of the sequence and which is reminiscent of various ATP, GTP or AMP glycine-rich loops. In this context, the conserved Arg (His in the E.coli enzyme) could be the AMP-binding residue.