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.