Biogenesis of Ribosomes

BIOGENESIS OF RIBOSOMES

The ribosomes are not self-replicating particles. The synthesis of various component of ribosomes such as rRNAs and proteins that are under genetic control,i.e.,rRNAs and mRNAs are transcribed by genes (DNA).The mechanism of biogenesis of the 70s and 80S ribosomes differ greatly,so can be studied separately as follows:

1.Biogenesis of 70S Ribosomes

The smith et al.,(1968)have suggested that in bacteria the RNA genes coding for the 5S,23S, and 16S ribosomal RNAs are tightly clustered in a region of the chromosomes.and are present in only a few copies.In other words,the ribosomal genes are in a single operon which is transcribed as a unit to synthesize a large molecule of RNA containing the 16S,23S, and 5S rRNA sequences.At first,rRNA is a greatly stabilized and protected by ribosomal proteins and thus any rRNA molecule that is synthesized is expected to have a far longer lifetime than other RNA species.Secondly, in contrast to other genes that may be transcribed only a few times or perhaps not all during the life of a cell,the RNA genes are in a state of perpetual transcription. Regulation of synthesis of 70S ribosomal proteins.For E.coli the task of coordination of synthesis of 54 ribosomal proteins and 3 rRNAs may be quite difficult.Since the synthesis of an excess of proteins would be wasteful and some of them were missing,ribosomes would not be assembled properly.Recent findings have indicated that control of synthesis of an exact amount of proteins at the proper time is achieved mainly through blocking the translation of mRNAs for ribosomal proteins when there is an excess of free ribosomal proteins.

For example,in E.coli mRNAs tend to be polycistronic,i.e,they contain the information for several proteins in a single mRNAs molecule.A transcriptional unit for multiple proteins is known as an operon and E.coli has six operons which contain genes for the ribosomal proteins. It has been shown that L4 ribosomal protein can inhibit the translation of several of ribosomal proteins from the polycistronic mRNA.The inhibition is overcome by addition of purified 23S rRNA that binds to protein L4.The synthesis of all ribosomal proteins from the induction of L4 protein overproduction by the longest operon.A ribosomal protein that involved in autogenous regulation of translation have one common property and they are primary or initial binding proteins so that they can bind directly to rRNA.

2.Biogenesis of 80S Ribosomes

The biogenesis of ribosome is more complex and involves a long lasting process in several regions of the cell are involved in eukaryotes.The 5.8S,18S, and 28S rRNAs are transcribed as much larger molecule in the nucleolar organizer that contains many copies 5.8S,18S and 28S rRNA genes or ribosomal DNA so that there is gene redundancy or amplification.In the nucleolus, that newly synthesized rRNAs accumulates and becomes associated,presumably by a self-assembly process,with 50 or more ribosomal proteins which have been synthesized in the cytoplasm by usual mechanism of protein synthesis and migrate to the cytoplasm of the cell,in the form of ribosomal subunits.

Biogenesis of 80S ribosomes involves in the following three main events:

1. Ribosomal of RNA synthesis by nucleolar organizer;
2. Biosynthesis of ribosomal proteins ;
3. 5S RNA synthesis

A.Ribosomal RNA synthesis inside nucleolus.The nucleolus is the response for the responsible for the synthesis of rRNAs obtained in 1964 when it was discovered an anucleate mutant of South African frog Xenopus laevis,unable to synthesizing RNA Brown and Gordon.In Xenopus chromosomes,the genes for 5S rRNA are found located in the telomeres.Direct evidence that the nucleolus is responsible for the synthesis of rRNA was obtained in 1964 when it was discovered that and a nucleolar mutant.

1.Ribosomal RNA genes.

The genes are tandemly repeated along the DNA molecule and separate from each other by stretches of spacer DNA. These rRNAs genes are actively transcribed and the nascent RNA chains are spread perpendicularly to the DNA axis.The spacer DNA can subdivide into a transcribed spacer so the latter is copied into RNA but does not rise to mature rRNA.Evidently, spacer DNA provides multiple functional binding sites that attract to activate the promoter.In electron microscopy,spacers have acted as sinks or storage areas for gene-specific binding proteins.

Gene amplification is the process in which a set of genes is selectively replicated.During pachytene, there is an active replication of the nucleolar organizers and at DNA is amplified 1000-fold.In Xenopus egg, 25 eg of extra DNA with 2,000,000Rrna genes is accommodated by between 1000 to 1500 nucleoli.In all the cases the amplified genetic material is not passed on the future cell generations.

2.Processing of rRNAs inside nucleolus.

As already described,rRNA genes are transcribed into a long precursor RNA.This precursor must be cleaved into 18S,28S, and 5.8 rRNA.In the cleavage process about 50 percent of the precursor RNA is regarded.In HeLa cell,the processing of rRNA involves the following steps:

5. The first ribosomal RNA in HeLa cells is a large 45S molecule of 14,000 nucleotides.Within this precursor molecule, the rRNAs are separated by stretches of spacer RNA and the order of transcription is 5’end _18S_5.8S_28S_3’end.On a fully active gene about 100 RNA polymerase my enzymes are transcribing simultaneously on the rRNA gene.
6. In nucleolus,45S RNA is rapidly methylated,even before transcription is completed.Methylations occur mostly on the ribose moiety and occur only in the 18S and 28S sequences that have to be conserved.Those segments of 45S which have to be degraded are not methylated.

• 45S RNA has a lifetime of about 15 minutes and is then cleaved into smaller components.

1. 20S RNA is rapidly processed in 18S rRNA and probably due to this reason,the small ribosomal subunits appear in the cytoplasm earlier than the large ribosomal subunits.The large ribosomal subunits have a slower RNA processing.
2. 32S RNA remains in the nucleolus for about 40 minutes and its then cleaved into 28R rRNA 5.8S rRNA .Both of these rRNAs present in the nucleolus for another 30 minutes before entering the cytoplasm as part of the large ribosomal subunits.Further, all these processing steps do not take place on naked RNA,but rather on RNA proteins complex.Ribosomal proteins bind to rRNA at the nucleolus and electron microscopic observation on “Christmas tree”spreads stained with antibodies against specific ribosomal proteins have shows that the primary or initial ribosomal proteins bind before the synthesis of 45S rRNA is completed.

B.Biosynthesis of ribosomal proteins

There is a great possibility that E.coli like translational regulation also exits in eukaryotes.Example the early embryo of frog is found to contain the mRNAs for all the 70 ribosomal proteins and except four of these mRNAs all are not translated until the midblastula stage when the synthesis of rRNAs is switched on in the nucleolus.The remaining four mRNAs are translated at all times.

C.5S RNA synthesis.

The 5S rRNA is synthesized from 20,000 genes in the oocytes but only from 400 genes in the somatic cells,which differ slightly in sequences.A gene for 5S rRNA contains an internal control region(ICR) in its middle region which is found essential for 5S transcription.To this control region of gene remains attached a special protein,called transcription factor III A which permits RNA polymerase III enzyme to recognize the promoter of a 5S rRNA gene.

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