endoribonuclease containing an essential RNA component (Chang and
Clayton 1987). RNase MRP is a ribonucleoprotein with its RNA component
and very likely all of its protein components encoded by nuclear
genes. The sequence of the nuclear gene for the RNA component of
RNase MRP has been obtained for vertebrates and fungi (Paluh and
Clayton 1995; for sequence comparisons of earlier reported sequences,
see Schmitt et a]. 1993), and the genes are distinctly similar. Inspection
of the coding region and surrounding sequences indicates that it is very
likely that these genes function as polymerase Wpolymerase I11 transcription
units, which may represent an ancient class of nuclear gene.
The genes are single copy in the cases of both mouse and human (Hsieh
et al. 1990), and it is interesting to note that there is extensive sequence
similarity in the 5 -flanking region that surpasses that typical for other
small RNA genes of nuclear origin. There are two protein components
identified for Saccharomyces cerevisiae RNase MRP: One is shared with
RNase P (Lygerou et al. 1994) and the other appears unique to RNase
MRP (Schmitt and Clayton 1994).