Polysomes are actively translating ribosomes whose purification can be performed on sucrose gradients. When (for example), translation initiation is defective there is a reduction in the number of ribosome translating a given mRNA and hence a reduction in the number of polysomes loading onto mRNA. Ultimately, it results in a decreased abundance of this mRNA species in the polysome-associated mRNA fraction.
Polysome-profiling is a methodology also known as translational profiling. It permits the identification of all cellular mRNAs whose translation status may be altered, for example, by mutations or environmental changes. This method couples polysomes fractionation with microarray technology. Total RNA and polysome-associated mRNA are purified from reference strains and strains to be tested and compared.
The clear genotype-phenotype association (S15 mutation-∆mt deletions in P. anserina) suggests that cytosolic ribosomes produced by the S15 mutant are defective to translate a subpopulation of cellular mRNA including those that encode proteins related to mtDNA metabolism. To identify these proteins we used the polysome-profiling methodology that is a genome-wide analysis designed to reveal the translatome of an organism.
This approach gave rise to a pool of responsive genes whose functional relations to mtDNA maintenance or mitochondrial biogenesis are currently characterized. Functional analyses of relevant genes uses a newly constructed molecular tool (Dequard-Chablat et al 2012).
The polysome-profiling approach was similarly used to characterize S. cerevisiae strains overexpressing the essential RRP5 gene implicated in nuclear rRNA maturation and whose over-expression facilitates mitochondrial import of a chimeric protein (Travaux en collaboration avec le Dr. Stéphane Le Crom, IBENS, Ens Paris)