7-2/MRP RNAs in plant and mammalian cells: association with higher order structures in the nucleolus.

Abstract

Mammalian MRP (for mitochondrial RNA processing) RNA, also known as 7‐2 RNA, is a nuclear encoded small RNA which has been reported to function in two different cellular compartments: in the mitochondria and in the nucleus. The ribonucleoprotein particle which contains the 7‐2/MRP RNA, called RNase MRP, has ribonucleolytic activity and shares some structural similarity with RNase P. It has been proposed that in mitochondria, the RNase MRP is responsible for endonucleolytic cleavage of primer RNA during DNA replication. We have characterized the gene and cDNAs encoding 7‐2/MRP‐like RNA in Arabidopsis and tobacco, and found that in plants this RNA is enriched in nucleoli but is undetectable in purified mitochondria isolated from tobacco leaves or cells grown in suspension. In glycerol gradients tobacco 7‐2/MRP RNA cosediments with large approximately 80S structures possibly representing ribosomal precursors. Fractionation of HeLa cells has also revealed that 7‐2/MRP resides in the nucleolus and that most of it is associated with complexes sedimenting at approximately 80S, similar to those containing the U3 nucleolar RNA which is known to participate in pre‐rRNA processing. These results indicate that the 7‐2/MRP ribonucleoparticle may be involved in ribosome biogenesis, in both plant and mammalian cells.

The family of box ACA small nucleolar RNAs is defined by an evolutionarily conserved secondary structure and ubiquitous sequence elements essential for RNA accumulation.

“…About half of the U64, U65, and U66 snoRNAs sedimented in 10S-15S monoparticlelike structures. The other half of these snoRNAs was found in large higher order structures that possess sedi mentation properties similar to the previously character ized ~80S nucleolar particles containing U3 snoRNA (Epstein et al 1984;Tyc and Steitz 1989;Kiss et al 1992).…”

“…Isolation of nuclei from human HeLa cells, and subfractionation of nuclei into nucleolar and nucleoplasmic fractions were per formed as described by Tyc and Steitz (1989) and Kiss et al (1992). Sonicates prepared from 5 x 10'' HeLa cells were loaded on a 10%-30% glycerol gradient and sedimented at 25,000 rpm for 10 hr at 4°C in a SW41 rotor (Beckman) as described by Tyc and Steitz (1989).…”

An upstream U-snRNA gene-like promoter is required for transcription of theArabidopsis thaliana7SL RNA gene

“…2 Taken together, these findings suggest that plant 7SL genes belong to the type 3 pol III-transcribed genes, which have their promoters situated upstream of the transcribed region. Furthermore, the 7SL upstream promoter is identical to the promoters present in the U-snRNA genes in plants (26,30,(37)(38)(39). In humans the 7SL RNA gene contains a type 4 promoter, with essential elements positioned both upstream and within the coding region (15)(16)(17)(18).…”

RNase MRP/RNase P: a structure‐function relation conserved in evolution?

“…Human and plant MRP/7-2 RNA were found in association with higher-order structures in the nucleolus [18]. These results led to the hypothesis that RNase MRP may be involved in RNA processing in the nucleolus [4,12,18,21a].…”