<p>The rihA gene encodes a ribonucleoside hydrolase that preferentially utilises cytidine and uridine [<cite idref="PUB00053958"/>]. The k_cat/K_M for uridine is approximately 10 times higher than for cytidine, largely due to a lower K_M value for uridine [<cite idref="PUB00053959"/>]. There are two other nucleoside hydrolases with differing specificities encoded by the rihB and rihC genes [<cite idref="PUB00053958"/>].An rihA null mutant can not use cytidine as a source of pyrimidine and the rihA and rihC genes are subject to catabolite repression [<cite idref="PUB00053958"/>].</p> <p>A crystal structure of RihA bound to the reaction product D-ribose has been solved at 1.8A resolution [<cite idref="PUB00053959"/>].</p> Pyrimidine-specific ribonucleoside hydrolase RihA