Respiratory NAD(P)H dehydrogenases of plants - Gene identity and expression in response to light and cold

Abstract: The respiratory chain of plants contains class 2 NAD(P)H dehydrogenases, not present in animals. The functional roles of these enzymes have been elusive for a long time. Two cDNAs, homologous to class 2 NADH dehydrogenase genes of yeast and E. coli, were isolated from potato leaves. The gene products, NDA1 and NDB1, were shown to reside on the internal and external sides of the inner mitochondrial membrane, respectively. As fusion proteins expressed in E. coli, the NDA1 protein was more firmly attached to membranes than the NDB1 protein. To study gene expression at the RNA level, a competitive quantitative RT-PCR system was developed. The nda1 gene was shown to be completely dependent on light. This light-dependence was seen at the mRNA, protein and activity levels. The nda1 gene was also seen to display a diurnal rhythm in potato leaves. As leaves develop from young to mature, the NDA1 transcript increases. This increase is reflected at the protein and activity levels. A real-time PCR system was developed for further transcript analyses. At cold-stress of potato plants, a decrease of NDA1 transcript was seen with consistent effects at the protein and activity levels. In the Arabidopsis genome, seven potential class 2 NAD(P)H dehydrogenase genes were identified. One of these genes, At-nda1 is regulated by light. The NDA1 enzyme is suggested to have a photorespiratory role, oxidizing excessive NADH in potato leaf mitochondria during photosynthesis.