|
Bacterial circadian rhythms, like other circadian rhythms, are endogenous "biological clocks" that have the following three characteristics: (a) in constant conditions (i.e. constant temperature and either constant light or constant darkness ) they oscillate with a period that is close to, but not exactly, 24 hours in duration, (b) this "free-running" rhythm is temperature compensated, and (c) the rhythm will entrain to an appropriate environmental cycle. Until the mid-1980s, it was thought that only eukaryotic cells had circadian rhythms. It is now known that cyanobacteria (a phylum of photosynthetic eubacteria) have well-documented circadian rhythms that meet all the criteria of bona fide circadian rhythms. In these bacteria, three key proteins whose structures have been determined (i) can form a molecular clockwork that orchestrates global gene expression and (ii) can reconstitute an oscillator in vitro. This system enhances the fitness of cyanobacteria in rhythmic environments. ==History: are prokaryotes capable of circadian rhythmicity?== Before the mid-1980s, it was believed that only eukaryotes had circadian systems.〔Johnson, C.H., S.S. Golden, M. Ishiura, and T. Kondo (1996) Circadian clocks in prokaryotes. Mole. Microbiol. 21: 5–11.〕 The conclusion that only eukaryotes have circadian oscillators seemed reasonable, because it was assumed that an endogenous timekeeper with a period close to 24 hours would not be useful to prokaryotic organisms that often divide more rapidly than once every 24 hours. The assumption might be stated as, "why have a timer for a cycle that is longer than your lifetime?" While intuitive, the conclusion was flawed. It was based on the assumption that a bacterial cell is equivalent to a sexually reproducing multicellular organism. However, a bacterial culture is more like a mass of protoplasm that grows larger and larger and incidentally subdivides. From this perspective, it is reasonable that a 24-hour temporal program could be adaptive to a rapidly dividing protoplasm if the fitness of that protoplasm changes as a function of daily alterations in the environment (light intensity, temperature, etc.). In 1985–86, several research groups discovered that cyanobacteria display daily rhythms of nitrogen fixation in both light/dark (LD) cycles and in constant light. The group of Huang and co-workers was the first to recognize clearly that the cyanobacterium ''Synechococcus'' sp. RF-1 was exhibiting circadian rhythms, and in a series of publications beginning in 1986 demonstrated all three of the salient characteristics of circadian rhythms described above in the same organism, the unicellular freshwater ''Synechococcus'' sp. RF-1.〔Huang T-C and Grobbelaar N (1995) The circadian clock in the prokaryote Synechococcus RF-1. Microbiology 141: 535–540.〕〔Lin R-F, and Huang, T-C (2009) Circadian rhythm of Cyanothece RF-1 (Synechococcus RF-1). Chapter 3 in: Bacterial Circadian Programs, J.L. Ditty, S.R. Mackey, C.H. Johnson, eds. (Springer), pp. 39–61.〕 Another ground-breaking study was that of Sweeney and Borgese,〔Sweeney BM, and Borgese MB (1989) A circadian rhythm in cell division in a prokaryote, the cyanobacterium Synechococcus WH7803. J. Phycol. 25: 183–186.〕 who were the first to demonstrate temperature compensation of a daily rhythm in the marine cyanobacterium, ''Synechococcus'' WH7803. Inspired by the research of the aforementioned pioneers, the cyanobacterium ''Synechococcus elongatus'' was genetically transformed with a luciferase reporter that allowed rhythmic gene expression to be assayed non-invasively as rhythmically "glowing" cells.〔Kondo, T., Strayer, C.A., Kulkarni, R.D., Taylor, W., Ishiura, M., Golden, S.S., and Johnson, C.H. (1993). Circadian rhythms in prokaryotes: luciferase as a reporter of circadian gene expression in cyanobacteria. Proc. Natl. Acad. Sci. USA 90, 5672–5676.〕〔Johnson, C.H., and Y. Xu (2009) The Decade of Discovery: How Synechococcus elongatus became a model circadian system 1990–2000. Chapter 4 in: Bacterial Circadian Programs, J.L. Ditty, S.R. Mackey, C.H. Johnson, eds. (Springer), pp. 63–86.〕 This system allowed an exquisitely precise circadian rhythm of luminescence to be measured from cell populations〔 and even from single cyanobacterial cells.〔Mihalcescu, I., Hsing, W., and Leibler, S. (2004). Resilient circadian oscillator revealed in individual cyanobacteria. Nature 430, 81–85.〕 The luminescence rhythms expressed by these transformed ''S. elongatus'' fulfilled all three key criteria of circadian rhythms: persistence of a 24-hour oscillation in constant conditions, temperature compensation, and entrainment. Thus, the work with various ''Synechococcus'' species firmly established that prokaryotic bacteria are capable of circadian rhythmicity, displacing the prior "no circadian clocks in prokaryotes" dogma. Nevertheless, persuasive evidence for circadian programs in bacteria other than the cyanobacteria is still lacking. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Bacterial circadian rhythms」の詳細全文を読む スポンサード リンク
|