October 15, 2012

Eukaryotic mRNA is modified during or after its transcription

  • Poly(A) is a stretch of ~200 bases of adenylic acid that is added to the 3 end of mRNA following its synthesis.
  • A eukaryotic mRNA transcript is modified in the nucleus during or shortly after transcription.
  • The modifications include the addition of a methylated cap at the 5 end and a sequence of poly(A) at the 3 end.
  • The mRNA is exported from the nucleus to the cytoplasm only after all modifications have been completed. 

The production of eukaryotic mRNA involves additional stages after transcription. Transcription occurs in the usual way, initiating a transcript with a 5 triphosphate end. However, the 3 end is generated by cleaving the transcript, rather than by terminating transcription at a fixed site. Those RNAs that are derived from interrupted genes require splicing to remove the introns, generating a smaller mRNA that contains an intact coding sequence.

Figure 5.16 shows that both ends of the transcript are modified by additions of further nucleotides (involving additional enzyme systems). The 5 end of the RNA is modified by addition of a "cap" virtually as soon as it appears. This replaces the triphosphate of the initial transcript with a nucleotide in reverse (3→5) orientation, thus "sealing" the end. The 3 end is modified by addition of a series of adenylic acid nucleotides [polyadenylic acid or poly(A)] immediately after its cleavage. Only after the completion of all modification and processing events can the mRNA be exported from the nucleus to the cytoplasm. The average delay in leaving for the cytoplasm is ~20 minutes. Once the mRNA has entered the cytoplasm, it is recognized by ribosomes and translated.

Figure 5.17 shows that the life cycle of eukaryotic mRNA is more protracted than that of bacterial mRNA. Transcription in animal cells occurs at about the same speed as in bacteria, ~40 nucleotides per second. Many eukaryotic genes are large; a gene of 10,000 bp takes ~5 minutes to transcribe. Transcription of mRNA is not terminated by the release of enzyme from the DNA; instead the enzyme continues past the end of the gene. A coordinated series of events generates the 3 end of the mRNA by cleavage, and adds a length of poly(A) to the newly generated 3 end.
Eukaryotic mRNA constitutes only a small proportion of the total cellular RNA (~3% of the mass). Half-lives are relatively short in yeast, ranging from 1-60 minutes. There is a substantial increase in stability in higher eukaryotes; animal cell mRNA is relatively stable, with half-lives ranging from 4-24 hours.
Eukaryotic polysomes are reasonably stable. The modifications at both ends of the mRNA contribute to the stability.