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

A tobacco pollen tube glycoprotein, p69 is encoded by the pollen-specific g ene ntp303 that is transcribed during pollen development and pollen tube gr owth, but it is abundantly translated only after pollen germination. To inv estigate the translational repression of ntp303 mRNA during pollen developm ent the compartmentation of ntp303 mRNA was examined and compared against a nother transcript (ntp52), which is efficiently translated during pollen ma turation. Three subcellular fractions were isolated: a post-polysomal fract ion enriched with messenger ribonucleoprotein particles, a polysomal fracti on and a novel fraction of EDTA/puromycin-resistant particles co-sedimentat ing with polysomes (EPP). At all developmental stages studied, ntp303 mRNA was found to be present in all fractions. Surprisingly, most of the transla tionally inactive ntp303 mRNA was localised in the polysomal fraction and E PPs, whereas ntp52 mRNA was distributed between the post-polysomal fraction and polysomes but was virtually undetectable in EPPs. This differential mR NA distribution pattern may help to explain the developmentally regulated t ranslational repression of the nty303 gene during pollen maturation, highli ghting a potential role of EPPs. A model of how this differential mRNA comp artmentation pattern regulates ntp303 mRNA translation is proposed.