The Sm binding site targets U7 snRNA to coiled bodies (spheres) of amphibian oocytes.

Using cytoplasmic and nuclear injection assays, we show that U7 snRNA constructs are targeted rapidly and specifically to the coiled bodies (spheres) in the germinal vesicle (GV) of the amphibian oocyte, including those coiled bodies attached to the lampbrush chromosomes at the histone gene loci. Because the U7 snRNP is required for removing the 3' end of histone pre-mRNA, we suggest that a major function of coiled bodies is to recruit U7 snRNPs to the histone gene loci, before they associate with the pre-mRNA. Targeting to coiled bodies requires the specific U7 Sm binding site; replacement of the U7 Sm site by that of U2 snRNA reduces this targeting dramatically. No other part of the molecule is required, and the U7 Sm binding site alone is sufficient to direct nuclear import of an unrelated RNA sequence and its specific targeting to coiled bodies. Injected U7 constructs displace the endogenous U7 in the coiled bodies, the amount of injected U7 that ends up in coiled bodies being roughly equal to the amount of endogenous U7 snRNA.     

Human genes encoding U3 snRNA associate with coiled bodies in interphase cells and are clustered on chromosome 17p11.2 in a complex inverted repeat structure.

Coiled bodies (CBs) are nuclear organelles whose morphological structure and molecular composition have been conserved from plants to animals. Furthermore, CBs are often found to co-localize with specific DNA loci in both mammalian somatic nuclei and amphibian oocytes. Much as rDNA sequences are called nucleolus organizers, we term these coiled body-associated sequences 'coiled body organizers' (CBORs). The only sequences that have been shown to be CBORs in human cells are the U1, U2 and histone gene loci. We wanted to determine whether other snRNA genes might also act as CBORs. In this paper we show that human U3 genes (the RNU3 locus) preferentially associate with CBs in interphase cells. In addition, we have analyzed the genomic organization of the RNU3 locus by constructing a BAC and P1 clone contig. We found that, unlike the RNU1 and RNU2 loci, U3 genes are not tandemly repeated. Rather, U3 genes are clustered on human chromosome 17p11.2, with evidence for large inverted duplications within the cluster. Thus all of the CBORs identified to date are composed of either tandemly repeated or tightly clustered genes. The evolutionary and cell biological consequences of this type of organization are discussed.     

New type of snRNP containing nuclear bodies in plant cells

In larch (Larix decidua Mill.) microspores a new type of nuclear bodies has been found which are an element of the spatial organization of the splicing system in plant cell. These are bizonal bodies, ultrastructurally differentiated into a coiled part and a dense part. Using immunocytochemistry and in situ hybridization at the EM level, the coiled part of the bizonal body was found to contain snRNA including U2 snRNA, Sm proteins and nucleolar proteins of the agyrophilic type and fibrillarin. The dense part contains Sm proteins but lacks snRNA. Such a separation of macromolecules related to splicing occurring within the bizonal bodies microspore is striking by the similarity of these bodies to amphibian oocyte snurposomes. The occurrence in plant cells, beside widely known coiled bodies (CBs), also of other nuclear bodies related to splicing proves that in plants similarly as for animals the differentiation among domains containing elements of the splicing system occurs.     

Characterization of the U3 and U6 snRNA genes from wheat: U3 snRNA genes in monocot plants are transcribed by RNa polymerase III

We have demonstrated recently that the genes encoding the U3 small nuclear RNA (snRNA) in dicot plants are transcribed by RNA polymerase III (pol III), and not RNA polymerase II (pol II) as in all other organisms studied to date. The U3 gene was the first example of a gene transcribed by different polymerases in different organisms. Based on phylogenetic arguments we proposed that a polymerase specificity change of the U3 snRNA gene promoter occurred during plant evolution. To map such an event we are examining the U3 gene polymerase specificity in other plant species. We report here the characterization of a U3 gene from wheat, a monocot plant. This gene contains the conserved promoter elements, USE and TATA, in a pol III-specific spacing seen also in a wheat U6 snRNA gene characterized in this report. Both the U3 and the U6 genes possess typical pol III termination signals but lack the cis element, responsible for 3-end formation, found in all plant pol II-specific snRNA genes. In addition, expression of the U3 gene in transfected maize protoplasts is less sensitive to -amanitin than a pol II-transcribed U2 gene. Based on these data we conclude that the wheat U3 gene is transcribed by pol III. This observation suggests that the postulated RNA polymerase specificity switch of the U3 gene took place prior to the divergence of angiosperm plants into monocots and dicots.