ORGAN-SPECIFIC STABILITY OF 2 LEMNA RBCS MESSENGER-RNAS IS DETERMINEDPRIMARILY IN THE NUCLEAR COMPARTMENT

It has previously been shown that the organ-specific expression of two members of the ribulose-1,5-bisphosphate carboxylaseloxygenase small subunit (rbcS) gene family is post-transcriptionally regulated in Lemn a gibba. While both small subunit genes encoding SSU1 and SSU5B were t ranscribed at comparable levels in root and frond nuclei, SSU1 mRNA ac cumulated to high levels in both roots and fronds in contrast to SSU5B mRNA, which was of very low abundance in the roots compared with the fronds. In this study, we have used two approaches to pinpoint the ste p(s) at which SSU1 and SSU5B mRNAs are differentially accumulated in t hese organs. In the first approach, total nuclear steady state mRNA wa s isolated from roots and fronds, and the amount of each transcript wa s measured by RNase protection assays and compared with the transcript ion rates in isolated nuclei. In the second approach, cordycepin was u sed to inhibit mRNA synthesis in Lemna fronds or roots, and the rate o f decay of each mRNA was measured by RNA gel blot analysis or RNase pr otection assays. Our findings indicate that the differential accumulat ion of SSU1 and SSU5B mRNAs in the fronds versus the roots is determin ed primarily in the nuclear compartment. In addition, SSU1 was found t o have a longer half-life in total steady state mRNA than SSU5B had in both organs. This feature probably accounts for SSU1 being the predom inantly expressed family member.