Biology Reference
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further lend support to the concept that lichen symbiosis is not a mutualistic association but a
predominantly of commensalism or even emphasizes the role of a parasite to the mycobiont. Joneson
et al . (2011) identifi ed the expression of 41 and 33 candidate genes by the fungal and algal partners,
respectively in Cladonia grayi lichen symbiosis. Proteins involved in self and non-self recognition,
lipid metabolism and negative regulation of glucose repressible genes are highly expressed in the
mycobiont whereas the phycobiont Asterochloris showed the up-regulation of chitinase-like protein,
an amino acid metabolism protein and a protein arginine methyltransferase.
viii) Specifi city of the partners : A number of investigators contributed to our understanding
of cyanobiont specifi city in cyanolichens. Traditional taxonomy helped in the identifi cation of
cyanobiont to be a species of Nostoc . The questions that have been addressed are whether: (i) there
is any diversity in the Nostoc strains that form symbiotic association or a single strain is invariably
represented in all cyanolichens, (ii) the same Nostoc strain forms association in a particular species
of cyanolichen in geographically distant regions, (iii) there is any diversity in the mycobiont that
forms symbiosis and (iv) morphological diversity of the lichen thallus is imposed by the photobiont
or mycobiont. To fi nd out strain differences in Nostoc , molecular markers have been of great help
in providing requisite answers.
The most widely used molecular markers for resolving strain differences of Nostoc and in
determining specifi city are tRNA Leu (UAA) intron and 16S rDNA sequences. The fungal-specifi c
markers are internal transcribed spacer (ITS) of the 5.8S gene of nuclear rDNA coding for RNAs
of the small and large subunits of the ribosome and intergeneric spacer (IGS) separating two
consecutive repeats. The 5.8S gene is homologous to a portion of the 23S gene of prokaryotes. It is
highly conserved, small in size and provides characters that are helpful in resolving differences in
between taxa (Hillis and Dixon, 1991). A brief account on the types of introns, their properties is
presented here.
Introns are defi ned as sequences of DNA that interrupt coding sequences of many genes and
also referred to as intervening sequences (Lewin, 2008). Depending on their structural and functional
features and nature of splicing mechanisms these are classifi ed into four groups: (i) spliceosomal
introns, (ii) group I introns, (iii) group II introns and (iv) archaeal introns (Belfort et al ., 1995; Nilsen,
2003). (i) Spliceosomal introns are the conventional introns present in eukaryotic cells. Along with
coding sequences, these are also transcribed into mRNA but are excised during the processing of
mRNA by spliceosomes (Logsdon, 1998). (ii) Group I introns can catalyze their own excision. They
are widely distributed in the genes of mitochondria, plastids, nuclear rRNA genes, bacterial tRNA
genes, genes of eukaryotes and viruses (Saldanha et al. , 1993). (iii) Group II introns differ from group
I introns in their splicing mechanism. Though they can catalyze their own excision, the splicing
mechanism resembles those of eukaryotes. They are exclusively found in genes coding for proteins,
tRNA and rRNA genes of organelles (Michel et al. , 1989; Saldanha et al ., 1993). (iv) Archaeal introns
are present in archaebacteria and these do not have the self-splicing mechanism of group I and
group II introns. These are spliced by an endonuclease that cuts at the exon-intron junction (Kjems
and Garrett, 1991; Lykke-Andersen et al ., 1997).
Xu et al . (1990) for the fi rst time reported a group I self-splicing intron in the gene for leucine
transfer RNA [tRNA Leu (UAA)] in two species of Anabaena , Anabaena sp. strain PCC 7120 and Anabaena
azollae . It is of 249 bp in the former and 291 bp in the latter. It is interesting to know that an intron of
similar nature and in the same identical position is present in the same tRNA gene of chloroplasts
of higher plants. This further strengthens the concept of endosymbiotic origin of chloroplasts.
The variability of intron in the tRNA Leu (UAA) gene in fi lamentous heterocystous cyanobacteria
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