Ribonucleoprotein particles

The Xenopus transcription factor IIIA not only acts as an essential RNA polymerase transcription factor for the expression of the 5S rRNA gene, it also binds to the 5S rRNA to form a 7S ribonucleoprotein particle that stabilizes the RNA until it is required for ribosome assembly and facilitates nuclear export of the 5S rRNA. Indeed, it was originally shown to be the protein component associated with 5S rRNA in the 7S particle in Xenopus oocytes before it was recognized as a transcription factor. How, we may ask, can this protein not only recognize specific DNA sequences in the 5S rRNA gene upstream region, but also recognize different, but equally specific, sequences in 5S rRNA ... 

Removal of introns. snRNP = small nuclear ribonucleoprotein particle. 

The spliceosome. The hnRNA of nuclei, which includes all of the pre-mRNA, is associated with proteins, which sometimes form very large 200S particles.604 After limited cleavage with nucleases they tend to sediment in the 30S-40S range and to contain a variety of proteins.605 606 Some of the proteins may have been involved in control of transcription.606 Others participate in splicing. The smaller snRNP particles then appear to come into the nucleus and displace much, but not all, of the protein present in the pre-mRNA ribonucleoprotein particles.605... 

Birnstiel, M. L., ed. (1988) Small Nuclear Ribonucleoprotein Particles, Springer, Vienna/ New York... 

RNA also differs from DNA in that there are not the same regularities in the overall composition of its bases and it usually consists of a single polynucleotide chain. There are different types of RNA, which fulfill different functions. About 80% of the RNA in a cell is located in the cytoplasm in clusters closely associated with proteins. These ribonucleoprotein particles specifically are called ribosomes, and the ribosomes are the sites of most of the protein synthesis in the cell. In addition to the ribosomal RNA (rRNA),... 

We have stated that translation begins before transcription is completed in prokaryotes. The situation is quite different in eukaryotes, where transcription and translation occur in different cellular compartments separated by the nuclear membrane. Large precursors of mRNA are synthesized in the nucleus these become complexed with proteins to form ribonucleoprotein particles which are modified and processed to form smaller mRNAs that become transported across the nuclear membrane to the cytoplasm. 

To assist in the modification and processing of mRNAs, eukaryotic cells contain in their nuclei small nuclear RNAs (snRNAs), which are complexed with specific proteins to form small nuclear ribonucleoprotein particles (snRNPs). These RNAs have been named U1, U2, U3,.. . , U13 and range in size from 100 to 220 bases. One, U3, is... 

Singh, O.P., Bjorkroth, B., Masich, S., Wieslander, L. and Daneholt, B. (1999) The intranuclear movement of Balbiani ring pre-messenger ribonucleoprotein particle. Exp. Cell Res., 251, 135-146. 

B1. Bachmann, M., Falke, D., Schroder, H. C., and Muller, W. E., Intracellular distribution of the La antigen in CV-1 cells after herpes simplex virus type 1 infection compared with the localization of U small nuclear ribonucleoprotein particles. J. Gen. Virol. 70, 881-891 (1989). 

The small nuclear ribonucleoprotein particles (snRNPs or snurps ) that carry out the splicing reaction use RNA-RNA basepairing to select the splice sites. Almost all intron-exon junctions contain the sequence AG-GU with the GU beginning the intron sequence. Furthermore, the consensus sequence for the beginning of the intron has a longer sequence complementary to the U1 RNA. Thus, assembly of the splicing complex, called the spliceosome, starts when the RNA component of the U1 snRNP base pairs with the junction between the 3 end of the exon and the 5 end of the intron. See Figure 12-13. 

The process of protein export involves a small, cytoplasmic ribonucleoprotein particle (the Signal Recognition Particle or SRP) with the signal coding mRNA sequence and/or the signal peptide itself. This interaction stops translation of the protein. Then, the stalled or arrested ribosome moves to the endoplasmic reticulum (ER). A receptor on the ER binds the SRP. 

Kedersha NL, Rome LH (1986) Isolation and characterization of a novel ribonucleoprotein particle large structures contain a single species of small RNA. J Cell Biol 103 699-709... 

Studies in this laboratory for the past several years have been concerned with the elucidation of the latter steps in this series of reactions. Specifically, efforts have been directed toward the characterization of the reaction involving the transfer of aminoacyl sRNA to mammalian ribonucleoprotein particles, the enzymatic and cofactor requirements, and possible intermediates in this process. The evidence obtained indicates that aminoacyl transfer is an enzymatic reaction requiring at least two enzyme fractions, which have been resolved and partially purified, GTP and a sulfhydryl compound further, the possibility exists that a ribosome-bound sRNA-amino acid (or peptide) compound is formed as an intermediate in this reaction. 

The transfer of labeled amino acids from aminoacyl sRNA to purified rat-liver ribonucleoprotein particles has been shown to require GTP, and a soluble portion (pH 5 Supernatant) of the cell. An enzyme fraction, aminoacyl transferase (or polymerase) I, purified from the pH 5 Supernatant was found to catalyze the transfer of amino acid to protein with microsomes, but not with the more purified ribonucleoprotein particles (ribosomes). When transferase I was supplemented with glutathione and a microsomal extract, microsomal aminoacyl transferase (or polymerase) H, transferring activity was restored. Since the pH 5 Supernatant was active in catalyzing the transfer of amino acids from sRNA to ribosomal protein, it was concluded that both transferring activities were present in this crude fraction. Resolution of the two activities from the pH 5 Supernatant fraction was obtained by salt-fractionation procedures. Neither enzyme fraction was active when incubated individually or with glutathione, but together in the presence of... 

Table 11.4 Small Nuclear Ribonucleoprotein Particles (snRNP) in Splicing of mRNA Precursors...

Small nuclear ribonucleoprotein particle Soil organic carbon store-operated channel Suppressors of cytokine signaling Superoxide dismutase CuZn-SOD enzyme (intracellular)... 

D, Keller W. Functions of the abundant U-snRNPs. In Small Nuclear Ribonucleoprotein Particles. Birnstiel ML, ed. 1988. Springer-Verlag, Berlin. 53. 

Pikielny CW, Rosbash M. Specific small nuclear RNAs are associated with yeast spliceosomes. Cell 1986 45 869-877. Frendewey D, Kramer A, Keller W. Different small nuclear ribonucleoprotein particles are involved in different steps of splicing complex formation. Cold Spring Harb. Symp. Quant. Biol. 1987 52 287-298. 

Konarska, M, Sharp PA. Interactions between small nuclear 81. ribonucleoprotein particles in formation of spliceosomes. Cell 1987 49 763-774. 

Grabowski PJ, Sharp PA. Affinity chromatography of splicing 82. complexes U2, U5, and U4 + U6 small nuclear ribonucleoprotein particles in the spliceosome. Science 1986 233 1294-1299. 

Table 28.3. Small nuclear ribonucleoprotein particles (snRNPs) in the splicing of mRNA precursors...

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