華岡 光正

In cyanobacteria, transcription of a set of genes is specifically induced by high-light-stress conditions. In previous studies, RpaB. a response regulator of the two-component system, was shown to he involved in this regulation in vitro and in vivo. In this study, we examined whether RpaB-dependent transcriptional regulation was extensively observed, not only under high-light-stress conditions but also under various light intensities. Transcription of high-light-dependent genes hliA, nblA and rpoD3 was transiently and drastically induced during a dark-to-light shift in a manner similar to high-lightstress responses. Moreover, expression of these genes was activated under various light-intensity upshift conditions. Phos-tag SDS-PA GE experiments showed that the phosphorylation level of RpaB was decreased along with transcriptional induction of target genes in all of the light environments examined herein. These results suggest that RpaB may be widely involved in transcriptional regulation under dark-to-light and light-intensity upshift conditions and that high-light-responsive genes may be required in various light conditions other than high-light condition. Furthermore, it is hypothesised that RpaB is regulated by redox-dependent signals rather than by high-light-stress-dependent signals.

A regular intake of plant-derived bioactive agents has gained popularity because of the health benefits. Fresh leafy greens, however, normally have a low concentration of such bioactive agents. In this study, we found that drying markedly affected the accumulation of secondary metabolites and that dried leaves of Perilla frutescens L. (perilla) contained more anticancer flavonoids than fresh leaves. Drying is a major method of food preparation, particularly for plant-based foods, but the quality of the bioactive agents contained in the fresh and dried leaves of perilla has received only scant attention. Quantitative analysis of the concentrations of perillaldehyde, rosmarinic acid, apigenin, luteolin, 4-hydroxyphenyllactic acid, and 4-coumaric acid, some of which are known as nutraceuticals, revealed that the effect of drying significantly increased apigenin (28-fold) and luteolin (86-fold), but decreased rosmarinic acid in all leaf stages. We examined the positive effect on flavonoid levels on perilla leaves and confirmed that, by comparison with fresh perilla leaves, the dried leaves contained greater concentrations of anticancer flavonoids regardless of variety, form, or manner of cultivation. This indicates that drying can significantly increase the level of flavonoids in perilla leaves without a loss of flavor. Therefore, drying is a simple and effective method to improve the concentrations of bioactive agents, which increases the intake of beneficial substances derived from herbs and edible plants. This finding serves as a method for the supply of raw plant materials rich in bioactive agents that are suitable for labeling as edible nutraceuticals.

α-Proteobacteria and cyanobacteria endosymbiosis has been crucial to the evolution of eukaryotic cells. The descendants of these bacteria gave rise to mitochondria and chloroplasts, and these organelles still retain their own genome proliferation systems. Coordination between the proliferation processes of these organelles and the eukaryotic cell cycle is indispensable for cellular maintenance, and we have studied this using the red alga Cyanidioschyzon merolae. During the cell cycle progression of C. merolae, organelle DNA replication (ODR) in both of the mitochondrion and the chloroplast occurs prior to nuclear DNA replication (NDR). We found that Mg-protoporphyrin IX (Mg-ProtoIX), a type of tetrapyrrole synthesized in the chloroplast, accumulates with the onset of ODR, thereby inducing NDR. Binding of the F-box protein Fbx3 to Mg-ProtoIX was also shown to be involved in the polyubiquitination of Cyclin 1, which activates cyclin-dependent kinase. Moreover, Mg-ProtoIX-Fbx3 binding inhibits Fbx3-mediated polyubiquitination of Cyclin 1. These results suggest that Fbx3 is a receptor for Mg-ProtoIX in the chloroplast signal to the nucleus and that it appears to function as a checkpoint for the coordination of ODR and NDR. In this chapter, we discuss the ODR and NDR coordination system in the cell cycle.