TRANSCRIPTS OF THE 2 NADPH PROTOCHLOROPHYLLIDE OXIDEREDUCTASE GENES PORA AND PORB ARE DIFFERENTIALLY DEGRADED IN ETIOLATED BARLEY SEEDLINGS

The light-dependent recduction of protochlorophyllide to chlorophyllid e in higher plants is catalyzed by two closely related enzymes, the NA DPH-Pchlide oxidoreductases A and B that are encoded by the nuclear ge nes PorA and PorB, respectively. The expression of the PorA gene is ne gatively regulated by light. It has formerly been reasoned that, apart from the well-studied transcriptional down-regulation, a post-transcr iptional mechanism may exist that contributes markedly to the light-in duced decline of PorA mRNA steady-state levels. We investigated the de gradation kinetics of the PorA messenger after inhibiting RNA synthesi s with cordycepin. The PorA mRNA was found to be inherently unstable. In contrast, the PorB mRNA was shown to be stabilized in the presence of cordycepin, suggesting degradation by a mechanism different from th at of PorA mRNA degradation. The PorA messenger instability is postula ted to be conferred by a previously described plant-specific DST eleme nt in its 3'UTR.