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Synthesis of Ribosomal RNA - rRNA

Synthesis of Ribosomal RNA - rRNA - The 70S ribosome of prokaryotes consists of a 308 subunit and a 50S subunit. The former contains 16S rRNA and the latter 238 and 58 rRNAs. In bacterial genes the sequences specifying 168, 238 and some times also5S RNA are arranged in a series mRNA is transcribed from DNA as a 30S unit, which bas been called p308.

Experimental evidence indicates that the 308 unit has the 16S component at the 5' end and the 238 component at the 3' end, with spacer units between the two components. In some prokaryotes 5S rRNA is transcribed at or near the 3' end.

During processing the p30S transcriptional unit is cleaved within the spacer segment by RNase III into 25S and l8S segments. These are then reduced to p23S and pl6S segments respectively, also by RNase III. Secondary trimming of these intermediates yields the final size, 23S and 168, respectively.

The enzyme involved in secondary trimming is probably a ribonuclease, designated as RNase M or maturase.In E. coli trimming of the pl6s component involves removal of a total of about 200 nucleotides from both 3' and 5' sides. The final cleavages occur when the rRNAs are associated with structural proteins of ribosomes in the 'preribosomal particles'.

Some nucleoside modifications take place in the p30S component, while others occur only after cleavage. Methylaiions found in 23S rRNA occur early at the p30S stage, whereas modifications found in 168 rRNA take place at a late stage of processing, in some cases after association with specific ribosomal proteins.

In E. coli the precursors of 58 rRNA are only a few nucleotides larger than mature SS rRNA. In Bacillus there are two types of larger precursors. One, designated as p5A, is 180 nucleotides long, while the other, P5B' is 140-150 nucleotides long. Both are apparently transcribed on different 5S genes.

Cleavage of p5A by the enzyme RNase M5 splits it into three components, a 5' terminal piece of about 20 nucleotides, mature 5S rRNA of 118 nucleotides and a 3' terminal piece of about 40 nucleotides. The 5' and 3' terminal pieces are broken down to their monouculeotides by an exonuclease, which thus serves a scavenging function.

Mature 5S rRNA is resistant to this enzyme. The precursor p5B is also split by RNase M5 into a 22 nucleotide 5' terminal piece, a mature 58 mRNA and some smaller 3' terminal pieces. In eukaryotes the 808 ribosome consists of 40S and 60S subunits. The 408 subunits contain 16-18S rRNA, and the 60S subunits 25-28S rRNA, S.88 RNAand.5S rRNA.

The genes coding for 16-188 rRNA and 25S-288 rRNA arc arranged in clusters of hundreds to thousands of copies. In eukaryotes the transcribed rRNA is 45S rRNA in mammals and 36-38S rRNA in lower eukaryotes, with molecular weights of 4.5 million and 2.6-2.8 million, respectively. There are currently two schemes for the possible arrangement of 28S and 18S segments within the 458 rRNA precursor; and the processing of the precursor rRNA.


(1) According to one scheme 458 rRNA has the general form: 5'P-28s rRNA - spacer - 18SRNA-spacer-3' OH. This transcriptional RNA has a life time of about 15 minutes during which methylation of the ribose moiety in the 288 and 188 regions takes place. An endonuclease cleaves 458 rRNA into 418 and 208components.

The cleavage takes place towards the 5' side of the 208 rRNA components. 41S rNA is degraded through 36S and 328 stages to mature 28S rRNA. During the three steps cleavage takes place in the non-methylated spacer segment by exonucleases. Similarly degration of the non methylated spacer region of the 20S component yields 18S rRNA.

(2) According to the second scheme the transcribed spacer sequence is situated at the 5'P end of 45S rRNA and the 28S com­ponent at the 3'OH end. The bulk of the experimental evidence supports this scheme which would also unify transcription in prokaryotes and eukaryotes. Processing occurs almost entirely within the nucleolus in eukaryote cells. The number of nucleoli within a cell depends on the extent to which the rRNA genes within a chromosome are dispersed.

45S transcriptional rRNA undergoes cleavage at four sites. Cleavage at site I removes the transcribe4 spacer sequence at the 5'P end. In primitive eukaryotes it may occur before transcription is completed. The order of cleavage at sites 2 and 3 varies in different species. Cleavage at site 2 separates 18S rRNA.

A segment of 140 nucleotides, with a sedimentation coefficient of 5.8S (formerly 78), situated between sites 3 and 4, remains covalently linked to the segment formed by .cleavage 3. In yeast tRNAs about 10% of 5.88 RNA is extended at the 5' end by 6-7 nucleotides. Cleavage 4 results in final trimming of the large rRNA segment. 58 rRNA is transcribed separately.

Analyses of the terminal sequences of 16-18S rRNA from many organisms reveal that the 3' end is almost always A-OH and the 5' end is pU. As many as 8 nucleotides at the 3' end are apparently uniform. They may be involved in the binding of mature rRNA to mRNA. Similarly 6 or more nucleotides may be uniform at the 5' end.

Nucleoside modifications take place on the initial transcript 45S rRNA in regions that give rise to mature 285 and 18S rRNAS. A few base methylations (6 in 18S rRNA) occur at later stages. This late methylation is characteristic of prokaryotes. Mefhy1ation seems to be essential for cleavage.
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