Yusuke Yoshida

The circadian system is composed of a number of feedback loops, and multiple feedback loops in the form of oscillators help to maintain stable rhythms. The filamentous fungus Neurospora crassa exhibits a circadian rhythm during asexual spore formation (conidiation banding) and has a major feedback loop that includes the FREQUENCY (FRQ)/WHITE COLLAR (WC) -1 and -2 oscillator (FWO). A mutation in superoxide dismutase (sod)-1, an antioxidant gene, causes a robust and stable circadian rhythm compared with that of wild-type (Wt). However, the mechanisms underlying the functions of reactive oxygen species (ROS) remain unknown. Here, we show that cellular ROS concentrations change in a circadian manner (ROS oscillation), and the amplitudes of ROS oscillation increase with each cycle and then become steady (ROS homeostasis). The ROS oscillation and homeostasis are produced by the ROS-destroying catalases (CATs) and ROS-generating NADPH oxidase (NOX). cat-1 is also induced by illumination, and it reduces ROS levels. Although ROS oscillation persists in the absence of frq, wc-1 or wc-2, its homeostasis is altered. Furthermore, genetic and biochemical evidence reveals that ROS concentration regulates the transcriptional function of WCC and a higher ROS concentration enhances conidiation banding. These findings suggest that the circadian system engages in cross-talk with the cellular redox state via ROS-regulatory factors.

A nucleoside diphosphate kinase‐1‐disrupted (ndk‐1^RIP‐1 ) mutant was observed to be defective in aerial hyphal and conidial development. In this study, two types of hyphae, fine and thick, were observed in wild‐type (Wt) strains. However, only fine‐type hyphae were observed in the ndk‐1^RIP‐1 mutants. The ndk‐1^RIP‐1 mutants were stimulated by oxidative stress and constitutively expressed an antioxidant enzyme catalase (CAT)‐3. Furthermore the ndk‐1^RIP‐1 mutants could form thick hyphae by oxidative stress and a disruption of cat‐3. These results suggest that the loss of thick hyphae in the ndk‐1^RIP‐1 mutants may be caused by the over‐expression of cat‐3.

Nucleoside diphosphate kinase (NDK) is an ubiquitous enzyme with the function of a signal transducer. In Neurospora crassa, an ndk-1P72H mutant carrying the point mutation Pro72His was isolated. We found that ndk-1P72H showed hypersensitivity to oxidative and heat stress and a decrease in the levels of catalase (Cat)-1 and -3 induced by oxidative, heat stress and illumination compared with wild type (Wt). We found, by conducting a yeast two-hybrid assay, that Cat-1 interacted with NDK-1. NDK-1 was suggested to control Cat-1 and Cat-3 at the post-transcriptional level in response to heat, oxidative stress and light