title: The initiation of nocturnal dormancy in Synechococcus as an active process.
abstract: BACKGROUND
  Most organisms, especially photoautotrophs, alter their behaviours in response to day-night alternations adaptively because of their great reliance on light. Upon light-to-dark transition, dramatic and universal decreases in transcription level of the majority of the genes in the genome of the unicellular cyanobacterium, Synechococcus elongatus PCC 7942 are observed. Because Synechococcus is an obligate photoautotroph, it has been generally assumed that repression of the transcription in the dark (dark repression) would be caused by a nocturnal decrease in photosynthetic activities through the reduced availability of energy (e.g. adenosine triphosphate (ATP)) needed for mRNA synthesis.
  
  
  RESULTS
  However, against this general assumption, we obtained evidence that the rapid and dynamic dark repression is an active process. Although the addition of photosynthesis inhibitors to cells exposed to light mimicked transcription profiles in the dark, it did not significantly affect the cellular level of ATP. By contrast, when ATP levels were decreased by the inhibition of both photosynthesis and respiration, the transcriptional repression was attenuated through inhibition of RNA degradation. This observation indicates that Synechococcus actively downregulates genome-wide transcription in the dark. Even though the level of total mRNA dramatically decreased in the dark, Synechococcus cells were still viable, and they do not need de novo transcription for their survival in the dark for at least 48 hours.
  
  
  CONCLUSIONS
  Dark repression appears to enable cells to enter into nocturnal dormancy as a feed-forward process, which would be advantageous for their survival under periodic nocturnal conditions.
authors.affiliation: Department of Electrical Engineering and Biological Science, Waseda University, TWIns, Shinjuku, Tokyo, 162-8480, Japan. sou.tacano@gmail.com.
authors.forename: Sotaro
authors.initials: S
authors.lastname: Takano
authors.affiliation: Department of Neuropharmacology, Graduate School of Pharmaceutical Sciences, Nagoya City University, Mizuhoku, Nagoya, 467-8603, Japan. tomita@phar.nagoya-cu.ac.jp.
authors.forename: Jun
authors.initials: J
authors.lastname: Tomita
authors.affiliation: Faculty of Education and Integrated Arts and Sciences, Waseda University, TWIns, Shinjuku, Tokyo, 162-8480, Japan. sonoike@waseda.jp.
authors.forename: Kintake
authors.initials: K
authors.lastname: Sonoike
authors.affiliation: Department of Electrical Engineering and Biological Science, Waseda University, TWIns, Shinjuku, Tokyo, 162-8480, Japan. hideo-iwasaki@waseda.jp.
authors.forename: Hideo
authors.initials: H
authors.lastname: Iwasaki
mesh_terms: D000255:Adenosine Triphosphate / Q000378:metabolism; D015964:Gene Expression Regulation, Bacterial; D017440:Photoperiod; D010788:Photosynthesis; D012329:RNA, Bacterial / Q000235:genetics; D046940:Synechococcus / Q000235:genetics / Q000502:physiology*; D014158:Transcription, Genetic
chemical_list: D012329:RNA, Bacterial; D000255:Adenosine Triphosphate