Synthesis of mRNA in pieces in VSG genes in trypanosome

It is well established that in eukaryotes, a precursor mRNA is first synthesized, and is processed to give rise to mRNA. In split genes also (Organization of Genetic Material 3.  Split Genes, Overlapping Genes and Pseudogenes), a pre-mRNA is first synthesized and intron regions are then excised to give rise to mRNA. However, in trypanosome (Trypanosoma brucei), it was shown that the mRNA for some glycoproteins synthesized by VSG gene.s, has a small segment at its 5' end which is synthesized as a discrete transcript separate from the remainder of mRNA. It was also shown that DNA encoding this 35-nucleotides leader RNA sequence (called 'mini exon') of all VSG mRNAs is separated by 30 kilobases from the DNA segment coding for the remaining mRNA. This 35-nucleotides sequence is common to all trypanosome mRNAs. No precursor giant mRNA, which could give rise to this mRNA by post-transcriptional splicing, could be observed. It was therefore, concluded that 35-nucleotides 'mini exon' RNA was synthesized as a part of a separate transcript and then either (i) joined to the remainder of the m-RNA-through trans-splicing or (ii) acted as a primer for the transcription of the longer segment of mRNA.

It has also been shown that mRNA can be produced from two precursor molecules through trans-splicing followed by joining. This became possible through the availability of a technique for in vitro splicing systems. In a study from Massachusetts Institute of Technology, USA, two artificial mRNA precursors could be synthesized, one having LI exon and the other having L2 exon. These two exons could be spliced together under optimum in vitro conditions to enable them to join together. In another study from Yale University, USA, chimeric precursors had either (i) the exon adE₁ and glE₂ or (ii) the exons adE₂ and glE₁. In each of these chimeric precursors, the two exons were connected by an intron and it was shown that splicing may be (i) cis leading to production of mRNA having exons from the same precursor or may be (ii) trans leading to the production of mRNA having exons from two precursors. The former is an intramolecular reaction and the latter is an intermolecular reaction and both were shown to proceed at the same rate.

In the above experiments, it was shown that the introns of two precursors have complementary sequences and, therefore, can be brought together either by base pairing or by mutual binding of a polypeptide. A scheme of cis and trans splicing of two chimeric precursor mRNAs to give rise to one mRNA is shown in Figure 37.20.