The translationally repressed pollen-specific ntp303 mRNA is stored in non-polysomal mRNPs during pollen maturation

Abstract 

A tobacco pollen tube glycoprotein, p69 is encoded by the pollen-specific gene ntp303 that is transcribed during pollen development and pollen tube growth, but it is abundantly translated only after pollen germination. To investigate the translational repression of ntp303 mRNA during pollen development the compartmentation of ntp303 mRNA was examined and compared against another transcript (ntp52), which is efficiently translated during pollen maturation. Three subcellular fractions were isolated: a post-polysomal fraction enriched with messenger ribonucleoprotein particles, a polysomal fraction and a novel fraction of EDTA/ puromycin-resistant particles co-sedimentating with polysomes (EPP). At all developmental stages studied, ntp303 mRNA was found to be present in all fractions. Surprisingly, most of the translationally inactive ntp303 mRNA was localised in the polysomal fraction and EPPs, whereas ntp52 mRNA was distributed between the post-polysomal fraction and polysomes but was virtually undetectable in EPPs. This differential mRNA distribution pattern may help to explain the developmentally regulated translational repression of the ntp303 gene during pollen maturation, highlighting a potential role of EPPs. A model of how this differential mRNA compartmentation pattern regulates ntp303 mRNA translation is proposed.