RNA components have been identified in preparations of RNase P from a number of eucaryotic sources, but final proof that these RNAs are true RNase P subunits has been elusive because the eucaryotic RNAs, unlike the procaryotic RNase P ribozymes, have not been shown to have catalytic activity in the absence of protein. We previously identified such an RNA component in Saccharomyces cerevisiae nuclear RNase P preparations and have now characterized the corresponding, chromosomal gene, called RPR1 (RNase P ribonucleoprotein 1). Gene disruption experiments showed RPR1 to be single copy and essential. Characterization of the gene region located RPR1 600 bp downstream of the URA3 coding region on chromosome V. We have sequenced 400 bp upstream and 550 bp downstream of the region encoding the major 369-nucleotide RPR1 RNA. The presence of less abundant, potential precursor RNAs with an extra 84 nucleotides of 5' leader and up to 30 nucleotides of 3' trailing sequences suggests that the primary RPR1 transcript is subjected to multiple processing steps to obtain the 369-nucleotide form. Complementation of RPR1-disrupted haploids with one variant of RPR1 gave a slow-growth and temperature-sensitive phenotype. This strain accumulates tRNA precursors that lack the 5' end maturation performed by RNase P, providing direct evidence that RPR1 RNA is an essential component of this enzyme.