Subtle determinants of the nucleocytoplasmic partitioning of in vivo-transcribed RNase MRP RNA in Xenopus laevis oocytes.

RNase MRP is a ribonucleoprotein originally identified on the basis of its ability to cleave RNA endonucleolytically from origins of mitochondrial DNA replication, rendering it a likely candidate for a role in priming leading-strand synthesis of mtDNA. In addition, a nuclear role for RNase MRP has been identified in yeast (Saccharomyces cerevisiae) ribosomal RNA processing. Consistent with a duality of function, RNase MRP has been localized to both mitochondria and nucleoli by in situ techniques. The RNA component of this ribonucleoprotein has been characterized from several different species. We previously cloned the gene for Xenopus laevis MRP RNA and showed that RNase MRP RNA is differentially expressed during amphibian development; in addition, the microinjected X. laevis RNase MRP RNA gene is correctly and efficiently transcribed in vivo. This article presents an analysis of the intracellular movement of in vivo-transcribed RNase MRP RNA in microinjected mature X. laevis oocytes. Although X. laevis MRP RNA is assembled into a ribonucleoprotein form and transported in an expected manner, human and mouse MRP RNAs exhibit markedly different transport patterns even though they are highly conserved in primary sequence. Furthermore, the only currently assigned protein (Th autoantigen) binding site in MRP RNA can be deleted without loss of nuclear export capacity. These results indicate that subtle determinants must exist for nucleocytoplasmic partitioning of this RNP and that the conserved Th autoantigen binding region appears unnecessary for the transit of in vivo-transcribed MRP RNA to the cytoplasm of mature X. laevis oocytes.