We will investigate the light regulation of the expression of two nuclear coded genes in Lemna. These genes are those for the small subunit of ribulose 1,5 bisphosphate carboxylase (RuBP Case) and the chlorophyll (chl) a/b protein. We will select cloned hybridization probes for these genes in order to study the light regulation in detail. We have shown that the level of translatable mRNAs for these two proteins can be regulated by phytochrome. We hope to demonstrate which step--transcription, post-transcriptional processing, or degradation of the mRNAs--is actually affected by phytochrome action. In addition, we shall investigate a second light-regulated step necessary for the effective synthesis of the chl a/b protein.