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In this article we will discuss about Heterogeneous Nuclear RNA (hnRNA).
Unlike prokaryotic mRNA, eukaryotic mRNAs are monocistronic. The primary transcript in eukaryotes is much larger than the mature mRNA and is called Heterogeneous nuclear RNA (hnRNA). It contains unique sequences and has about 10 times as many sequences as the mature mRNA. hnRNA undergoes processing and finally the mRNA is produced and therefore, it is called “mRNA precursor” or “pre-mRNA”.
Main characteristics of hnRNA are as follows:
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(i) Size is not uniform and sedimentation values range from 20S to 100S.
(ii) Base composition of hnRNA is similar to DNA from which it is transcribed.
(iii) Mostly the hnRNA is located outside the nucleolus.
(iv) mRNA is derived from hnRNA through enzymatic processing.
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The hnRNA undergoes the following chemical changes during processing.
1. Packaging of hnRNA with protein:
Newly synthesized hnRNAs are packed with protein to form a ribonucleoprotein complex. Protein particles are 30S particles and they are distributed on the hnRNA.
2. Polyadenylation or Poly (A) tail formation:
After transcription, poly (A) tail is added at the 3′-end, the polymerization reaction is catalyzed by the enzyme “poly (A) polymerase”. This enzyme does not require DNA template and adds about 200 adenine nucleotides at the 3′ OH- end of hnRNA.
Poly (A) tail functions in the transport of mRNA from the nucleus to the cytoplasm. It also serves to slow down the intracellular degradation of mRNA. However, poly (A) tail is not found in all the eukaryotic mRNAs, e.g., histone mRNA.
3. RNA splicing:
The hnRNA consists of coding sequences (exons) and noncoding sequences called “intervening sequences” (introns). These sequences are found alternating to each other; such genes are called split genes. The number of exons and introns differ in different genes (Table 4.2). For example, in chicken, the gene actin contains 4 exons and 3 introns, while the gene collagen contains 51 exons and 50 introns.
In extreme cases, the actual coding region of mRNA is about 10% of the length of hnRNA. During processing, the introns are cleaved and exons are united in the correct sequence, this process is called RNA splicing. Some ribonucleoprotein particles known as “snurps” are involved in removal of introns and splicing of RNA.
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Base sequences at the end of intron are similar in all the introns and are called “splice junctions”. The RNA of the “snurp” has complementary sequences to “splice junctions” and it is involved in the process of intron removal. However, all the genes do not require the snurps for processing of RNA. Self-spicing of RNA occurs in some mitochondrial genes and ribosomal RNA genes in ciliated protozoa.
4. 5′ Capping:
During the processing of RNA, a “cap” is formed at the 5′-end (Fig. 4.3, 4.4). The cap consists of one or two nucleotides whose sugar is methylated at the 2′ position. The methylated nucleotide is connected by a triphosphate bridge to 7-methylguanosine (m7G). The 7- methylguanosine is associated with 5′-triphosphate at the 5′-end of mRNA and thus it terminates into 3′-OH end. (Fig. 4.3).
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The terminal modified base (m7G-PPP) of the cap prevents the mRNA from attack and degradation by 5′-exonuclease. The eukaryotic mRNAs, unlike prokaryotic mRNAs, are long lived molecules. They can survive from hours to days. In dormant seeds, they survive for years. Mature mRNA is transported to cytoplasm.
Mature mRNA:
Mature mRNA contains 5 parts (Fig. 4.4):
(i) One central coding segment,
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(ii) Two un-translated segments, one on each side of the coding central segment,
(iii) 7-methylguanosine linked by a triphosphate linkage at the 5′-end, and
(iv) Poly(A) tail, about 200 bases long, at the 3′-OH end.
The length of the mRNA, coding region and un-translated segments very in different genes. For example, the a-globin gene in human has 575 bases in total mRNA. The coding region has 429 bases, while the un-translated segments at 5′-end and 3′-end are 37 and 109 bases long, respectively.
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The chick ovalbumin mRNA contains 1859 bases of which 1164 bases make the coding segment, 64 bases at 5′-end and 631 bases at 3′-end constitute the two un-translated segments.
