“ INTRON INDUCED GENETIC VARIATION”

 UNIVERSITY OF AGRICULTURAL SCIENCES, RAICHUR

Department of Genetics and Plant Breeding
College of Agriculture, Raichur
Master’s Seminar-I
on

“ INTRON INDUCED GENETIC VARIATION”

Introns are the intervening sequences that are non coding regions since they don’t seem to
code for any enzymes or structural proteins. The pre-mRNA strand is processed so that its
introns are removed and the exons are joined together to make a mature mRNA this process is
called RNA splicing (Sharp, 1993). The two boundaries between an intron and two exons
located at its two ends are called splicing junctions. Spliceosomes are composed of a mixture of
protein and small RNA molecules, which remove introns from the pre mRNA. Alternative
patterns of pre-mRNA splicing that produced different mature mRNAs containing various
combinations of exons from a single precursor mRNA. Alternative splicing generates a
tremendous amount of proteomic diversity in humans and significantly affects various functions
in cellular processes, tissue specificity, developmental states and disease conditions.
Alternative splicing (AS) is a major contributor to transcriptome and proteome diversity.
Evolutionary studies help to address questions that are fundamental to understanding this
important process. The main mechanism for exon selection in higher eukaryotes is exon
definition: the splicing machinery is placed across exons, constraining their length. Early
eukaryotic ancestors are rich in introns, contain degenerate splicing signals and complex
spliceosomes, and share homology of splicing factors in different species. These observations
suggest an early eukaryotic origin of AS. There are three known evolutionary mechanisms that
could account for the appearance of an alternatively spliced exon: exon shuffling (a form of gene
duplication), exonization of intronic sequences and transition of a constitutive exon to an
alternative exon. The formation of an alternative exon permits new functions to be established
without eliminating the original function of the protein. Alu elements — primate-specific
reteroelements — substantially contribute to the creation of new alternative exons, which can
enhance the genomic repertoire. (Hadas et al., 2010)

A large variety of monocot and dicot intron-containing fragments inserted in the 5'
untranslated leader, between the CaMV 35S promoter and the uidA gene (coding for the l~-
glucuronidase: GUS). Relative strengths of the intron-containing ~ fragments were evaluated by
comparing transient GUS expression after particle bombardment in embryogenic maize and
bluegrass suspension cultures. Results were confirmed a dramatic dependence on the presence of
an intron for chimeric gene expression in both species. On average, the maize first intron of ubil
provided the highest enhancement of gene expression in maize and bluegrass (71- and 26-fold
enhancement, respectively). Half of the introns tested affected gene expression differently in

bluegrass and maize. This suggests that the intron-mediated enhancement of gene expression
generally obtained with maize may not be fully applicable to all monocots. They also reported
that enhancement of gene expression (92-fold) in a monocot species by a dicot intron. (Philippe
et al., 1996).
The genomic clone encoding an a-tubulin, OsTubA1, has been isolated from rice (Oryza
sativa L.). The gene consists of four exons and three introns. RNA-blot analysis showed that the
gene was strongly expressed in actively dividing tissues, including root tips, young leaves, and
young flowers. Analysis of chimeric fusions between OsTubA1 and b-glucuronidase (GUS)
revealed that the intron 1 was required for high-level GUS expression in actively dividing
tissues, corresponding with normal expression pattern of OsTubA1. Fusion constructs lacking
the intron 1 showed more GUS staining in mature tissues rather than young tissues. When the
intron 1 was placed at the distal region from 59-upstream region or at the 39- untranslated
region, no enhancement of GUS expression was observed. Sequential deletions of the OsTubA1
intron 1 brought about a gradual reduction of GUS activity in calli. These results suggest that
tissue-preferential expression of the OsTubA1 gene is mediated by the intron 1 and that it may be
involved in a mechanism for an efficient RNA splicing that is position dependent. (Jong et al.,
2000).

REFERENCES:

Hadas, K., Galit, L. M. and Gil, 2010. Alternative splicing and evolution: diversification, exon

definition and function, Nat. Rev. Genet., 11:345-355.

Jong, S. J., Sichul, L., Hong, J., Sung, H. J., Chanhong, K. and Gynheung, 2000. Tissue-

preferential expression of a rice a-tubulin gene OsTubA1 mediated by the first intron1.

Plant Physiol., 143: 1005-1014.

Philippe, V., Kim, R., Finer, Engler, D. E., Richard, C. and John, J. F, 1996. Intron-mediated

enhancement of gene expression in maize (Zea mays L.) and bluegrass (Poa pratensis

L.). Plant Cell Rep., 15:489-494.

Sharp, P., 1993. Split genes and RNA splicing, Cell, 91: 875-879.