RNA synthesis and Processing

The differences in transcription elongation of prokaryotes and eukaryotes

Eukaryotes:

-          Transcription elongation in eukaryotes is tightly coupled to RNA processing.

-          The RNA splicing removes intron sequences from newly transcribed pre-mRNA and join the exons (coding regions) together.

-          The splicing process of RNA is catalyzed by spliceosome. Sometimes, alternative splicing happens, allows differential expression of exons .

-          The mRNa is capped and polyadenylated in the nucleus and then exported to the cytoplasm for translation.

-          The nucleolus is a ribosome-producing factory.

-          Bote: although intron will be removed during the process of splicing, but it is very important because it controls many process in transcription.

Prokaryotes:

-          Both transcription and translation occur in the same compartment. There are no introns to splice out and the mRNA is not capped or polyadenylated.

The sequence required for intron removal in eukaryotic RNA and the 3 major steps in the RNA splicing reaction

-    In the mRNA strand, especially in the intron sequence, the adenine initiates its attack on 5’ splice site (the upstream of the intron sequence).

-    3’-OH of upstream exon reacts with 5’ of the downstream exon.

-    Exons are joined, and the lariat is released.

-    In the intron, the 5’ upstream has GU invariant bases while the 3’downstrem has AG invariant bases

The major steps in generating the 3’ end of eukaryotic mRNA

As RNA pol II reaches the end of a gene, the position of the 3’end of each mRNA molecule is ultimately specified by a signal encoded in the genome. These signals are transcribed into RNA as the RNA pol II moves through them, and they are then (as RNA) by a series of RNA-binding proteins and RNA-processing enzymes.  Two multisubunit proteins, CPSF and CstF are part of the RNA pol tail, and then transferred to the 3’end processing sequence on the RNA.

Once these proteins bind to the specific nucleotides, additional proteins assemble with them to create the 3’end of the mRNA. First, the RNA is cleaved. Next poly-A pol enzymes adds, one at a time, nucleotides to the 3’end produced by the cleavage without the requirement of template ( Note that: poly-A pol is not directly encoded in the genome).

As the poly-A tail is synthesized, proteins called poly-A-binding protein assemble onto the poly-A tail and remain their bound to direct mRNA to travel from the nucleus to cytosol as well we the synthesis of a protein on the ribosome.