RRNA processing

The most abundant protein in the fibrillar regions of the nucleus, where the early stages of pre-rRNA processing occur, is fibrillarin.541,548,549 Many of the snoRNAs are closely associated with this protein. Fibrillarin is also well known as an autoantigen, which can induce formation of destructive antibodies that cause scleroderma (Chapter 31, Section F). 

In the literature it is reported that in E. coli mutations in the rRNA dimethylase ksgAp block the m62Am62A dimethylation of 16S rRNA they do not, however, interfere with rRNA processing. Consequently, the mutants synthesize SSU rRNAs lacking the modification. These allow the growth of the mutant strains at a reduced... 

In both eukaryotic and prokaryotic cells, large precursors to rRNA are transcribed and processed to produce the mature rRNAs. Processing involves methylation of bases and/or ribose and endolytic cleavage to cut out unwanted sequences. The eukaryotic products are the 18, 5.8, and 28 S rRNAs. In prokaryotes, the final products are the 16 S rRNA, a spacer region that includes one or two tRNAs, the 23 S rRNA, and the 5 S rRNA and, in some instances, one or two additional tRNAs. 

Processing of rRNA and tRNA from Precursor Transcripts rRNA Processing... 

Figure 25.7 shows the processing pathway of human 45S rRNA into the two functional 28S rRNA and 18S rRNA products. As described in Chapter 24, eukaryotic rRNA genes are transcribed by RNA pol I. Less is known about the enzymology of eukaryotic rRNA processing than that of bacteria however, it is presumed that site-specific RNases are also involved in the cleavage reactions. 

MuJtistep rRNA processing [discard S80D nucleotides)... 

The most abundant protein in the fibrillar regions of the nucleus, where the early stages of pre-rRNA processing occur, is Many of fhe... 

Which noncoding RNA is responsible for recruiting RNA-modifying proteins that are necessary for rRNA processing ... 

Prokaryotic pre-rRNA processing The E. coli primary rRNA transcript contains 16S rRNA, 1 or 2tRNA, 23S rRNA and 5S rRNA and some with 1 or 2 more tRNA (in order from 5 to 3 ends). The trimming and processing of pre-rRNA to mature rRNA involves various RNases such as RNase HI, RNase P, RNase E, RNase F and RNase M at different stages. 

Eukaryotic pre-rRNA processing The primary rRNA transaipt is 45S RNA containing 18S, 5.8S and 28S rRNA separated by spacer sequences (in order from 5 to 3 ends). An initial methylation of -110 sites yields O -methyribose residues (-80% of methylated sites) and methylated bases (-20% of methylated sites) such as N , N -dimethy-ladenine and 2-methylguanine. Subsequent trimming and processing by RNases produces mature rRNA. 

Mougey, E O Reilly, M Osheim, Y. N Miller, O. L.. Jr.. Beyer, A. L., and Sollner-Webb, B. (1993). The terminal balls characteristic of eukaryotic rRNA transcription units in chromatin spreads are rRNA processing complexes. Genes Dev. 7, 1609-1619. 

Peculis, B. A., and Steitz, J. A. (1993). Disruption of U8 nucleolar snRNA inhibits 5.8S and 28S rRNA processing in the Xenopus oocyte. Cell (Cambridge, Mass.) 73(6), 1233-1245. 

---