Transcripts of the two NADPH protochlorophyllide oxidereductase genes PorA and PorB are differentially degraded in etiolated barley seedlings.

The light-dependent reduction of protochlorophyllide to chlorophyllide in higher plants is catalyzed by two closely related enzymes, the NADPH-Pchlide oxidoreductases A and B that are encoded by the nuclear genes PorA and PorB, respectively. The expression of the PorA gene is negatively regulated by light. It has formerly been reasoned that, apart from the well-studied transcriptional down-regulation, a post-transcriptional mechanism may exist that contributes markedly to the light-induced decline of PorA mRNA steady-state levels. We investigated the degradation kinetics of the PorA messenger after inhibiting RNA synthesis 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 that of PorA mRNA degradation. The PorA messenger instability is postulated to be conferred by a previously described plant-specific DST element in its 3'UTR.