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Cell Division Cycles and Circadian Rhythms

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Book cover Bacterial Circadian Programs

Abstract

Cell division cycles and circadian clocks are major periodic processes in living organisms. Circadian rhythms of cell division have been found in many eukaryotes and some prokaryotes. Circadian clocks gate cell division within discrete time windows. Among bacteria, circadian clocks have been found only in cyanobacteria. The freshwater unicellular cyanobacterium Synechococcus elon-gatusM PCC 7942, which utilizes light as an energy source, grows and divides in the daytime and stops dividing during the night. When the cells are placed in continuous light conditions, the rhythmic occurrence of cell division continues with a period of ̃24 h. Whether the cyanobacterial cells are rapidly growing or halted in their division cycle, the circadian clock appears to tick steadily and operates normally. This phenomenon implies an independence of circadian timekeeping from the cell division cycle. The mechanisms and significance of circadian control of the cell division cycle in cyanobacteria are discussed here.

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References

  • Angert ER (2005) Alternatives to binary fission in bacteria. Nat Rev Microbiol 3:214–224

    Article  PubMed  CAS  Google Scholar 

  • Aronson BD, Johnson KA, Loros JJ, Dunlap JC (1994) Negative feedback defining a circadian clock: autoregulation of the clock gene frequency. Science 263:1578–1584

    Article  PubMed  CAS  Google Scholar 

  • Asato Y (1984) Characterization of cell cycle events in synchronized cultures of Anacystis nidu-lans. J Gen Microbiol 130:2535–2542

    CAS  Google Scholar 

  • Asato Y (2003) Toward an understanding of cell growth and the cell division cycle of unicellular photoautotrophic cyanobacteria. Cell Mol Life Sci 60:663–687

    Article  PubMed  CAS  Google Scholar 

  • Barák I, Wilkinson AJ (2007) Division site recognition in Escherichia coli and Bacillus subtilis. FEMS Microbiol Rev 31:311–326

    Article  PubMed  CAS  Google Scholar 

  • Barnett A (1969) Cell division: a second circadian clock system in Paramecium multimicronuclea-tum. Science 164:1417–1419

    Article  PubMed  CAS  Google Scholar 

  • Binder BJ, Chisholm SW (1990) Relationship between DNA cycle and growth rate in Synechococcus sp. strain PCC 6301. J Bacteriol 172:2313–2319

    PubMed  CAS  Google Scholar 

  • Bonneau R, Facciotti MT, Reiss DJ, Schmid AK, Pan M, Kaur A, Thorsson V, Shannon P, Johnson MH, Bare JC, Longabaugh W, Vuthoori M, Whitehead K, Madar A, Suzuki L, Mori T, Chang DE, Diruggiero J, Johnson CH, Hood L, Baliga NS (2007) A predictive model for transcrip-tional control of physiology in a free living cell. Cell 131:1354–1365

    Article  PubMed  CAS  Google Scholar 

  • Bouget F Y, Moulager M, Corellou F (2008) Circadian regulation of cell division. In: Verma DP, Hong Z (eds) Cell division control in plants. Springer, Heidelberg, pp 3–12

    Chapter  Google Scholar 

  • Bré MH, Ferjani EE, Lefort-Tran M (1981) Sequential protein dependent steps in the cell cycle. Initiation and completion of division in vitamin B12 replenished Euglena gracilis. Protoplasma 108:301–318

    Article  Google Scholar 

  • Bruce VG, Bruce NC (1981) Circadian clock-controlled growth cycle in Chlamydomonas rein-hardi. In: Schweiger HG (ed) International cell biology, 1980–1981. Proc 2nd Int Congr Cell Biol, Berlin, 1980. Springer, Berlin, pp 823–830

    Google Scholar 

  • Chabot JR, Pedraza JM, Luitel P, van Oudenaarden A (2007) Stochastic gene expression out-of-steady-state in the cyanobacterial circadian clock. Nature 450:1249–1252

    Article  PubMed  CAS  Google Scholar 

  • Chen T-H, Chen T-L, Hung L-M, Huang T-C (1991) Circadian rhythm in amino acid by Synechococcus RF-1. Plant Physiol 97:55–59

    Article  PubMed  CAS  Google Scholar 

  • Chisholm SW, Morel FMM, Slocum SW (1980) The phasing and distribution of cell division in cycles in marine diatoms. In: Falkowski P (ed) Primary productivity and biogeochemical cycles in the sea. Plenum, New York, pp 281–300

    Google Scholar 

  • Cremer C, Cremer T, Zorn C, Zimmer J (1981) Induction of chromosome shattering by ultraviolet irradiation and caffeine: comparison of whole-cell and partial-cell irradiation. Mutat Res 84:331–348

    PubMed  CAS  Google Scholar 

  • Domain F, Houot L, Chauvat F, Cassier-Chauvat C (2004) Function and regulation of the cyano-bacterial genes lexA, recA and ruvB: LexA is critical to the survival of cells facing inorganic carbon starvation. Mol Microbiol 53:65–80

    Article  PubMed  CAS  Google Scholar 

  • Dvornyk V, Vinogradova O, Nevo E (2003) Origin and evolution of circadian clock genes in prokaryotes. Proc Natl Acad Sci USA 100:2495–2500

    Article  PubMed  CAS  Google Scholar 

  • Edmunds LN Jr (1966) Studies on synchronously dividing cultures of Euglena gracilis Klebs (strain Z). III. Circadian components of cell division. J Cell Physiol 67:35–43

    Article  PubMed  Google Scholar 

  • Edmunds LN (1988) Cellular and molecular bases of biological clocks. Springer, New York

    Google Scholar 

  • Edmunds LN Jr, Adams KJ (1981) Clocked cell cycle clocks. Science 211:1002–1013

    Article  PubMed  CAS  Google Scholar 

  • Edmunds LN Jr, Funch RR (1969) Circadian rhythm of cell division in Euglena: effects of random illumination regimen. Science 165:500–503

    Article  PubMed  Google Scholar 

  • Ehret CF (1980) On circadian cybernetics, and the innate and genetic nature of circadian rhythms. In: Scheving LE, Halberg F (eds) Chronobiology: principles and applications to shifts in schedules. Springer, Heidelberg, pp 109–126

    Google Scholar 

  • Ehret CF, Wille JJ (1970) The photobiology of circadian rhythms in protozoa and other eukaryotic microorganisms. In: Halldal P (ed) Photobiology of microorganisms. Wiley, London, pp 369–416

    Google Scholar 

  • Fu L, Lee CC (2003) The circadian clock: pacemaker and tumour suppressor. Nat Rev Cancer 3:350–361

    Article  PubMed  CAS  Google Scholar 

  • Fu L, Pelicano H, Liu J, Huang P, Lee C (2002) The circadian gene Period2 plays an important role in tumor suppression and DNA damage response in vivo. Cell 111:41–50

    Article  PubMed  CAS  Google Scholar 

  • Garrido T, Sánchez M, Palacios P, Aldea M, Vicente M (1993) Transcription of ftsZ oscillates during the cell cycle of Escherichia coli. EMBO J 12:3957–3965

    PubMed  CAS  Google Scholar 

  • Goto K, Johnson CH (1995) Is the cell division cycle gated by a circadian clock? The case of Chlamydomonas reinhardtii. J Cell Biol 129:1061–1069

    Article  PubMed  CAS  Google Scholar 

  • Graumann PL (2007) Cytoskeletal elements in bacteria. Annu Rev Microbiol 61:589–618

    Article  PubMed  CAS  Google Scholar 

  • Grobbelaar N, Huang TC, Liu H Y, Chow TJ (1986) Nitrogen-fixing endogenous rhythm in Synechococcus RF-1. FEMS Microbiol Lett 37:173–177

    Article  CAS  Google Scholar 

  • Halberg F, Conner RL (1961) Circadian organization and microbiology: variance spectra and a periodogram on behavior of Escherichia coli growing in fluid culture. Proc Minn Acad Sci 29:227–239

    Google Scholar 

  • Halberg F, Cornélissen G (1991) The spectrum of rhythms in microorganisms revisited. Chronobiologia 18:114

    Google Scholar 

  • Halberg F, Cornélissen G, Katinas G, Syutkina EV, Sothern RB, Zaslavskaya R, Halberg F, Watanabe Y, Schwartzkopff O, Otsuka K, Tarquini R, Frederico P, Siggelova J (2003) Transdisciplinary unifying implications of circadian findings in the 1950s. J Circadian Rhythms 1:2

    Article  PubMed  Google Scholar 

  • Hardin PE, Hall JC, Rosbash M (1990) Feedback of the Drosophila period gene product on cir-cadian cycling of its messenger RNA levels. Nature 343:536–540

    Article  PubMed  CAS  Google Scholar 

  • Harmer SL, Panda S, Kay SA (2001) Molecular bases of circadian rhythms. Annu Rev Cell Dev Biol 17:215–253

    Article  PubMed  CAS  Google Scholar 

  • Harry E, Monahan L, Thompson L (2006) Bacterial cell division: the mechanism and its precison. Int Rev Cytol 253:27–94

    Article  PubMed  CAS  Google Scholar 

  • Herdman M, Faulkner BM, Carr NG (1970) Synchronous growth and genome replication in the blue-green alga Anacystis nidulans. Arch Mikrobiol 73:238–249

    Article  CAS  Google Scholar 

  • Hiraga S (2000) Dynamic localization of bacterial and plasmid chromosomes. Annu Rev Genet 34:21–59

    Article  PubMed  CAS  Google Scholar 

  • Holtman CK, Chen Y, Sandoval P, Gonzales A, Nalty MS, Thomas TL, Youderian P, Golden SS (2005) High-throughput functional analysis of the Synechococcus elongatus PCC 7942 genome. DNA Res 12:103–115

    Article  PubMed  CAS  Google Scholar 

  • Holtzendorff J, Partensky F, Jacquet S, Bruyant F, Marie D, Garczarek L, Mary I, Vaulot D (2001) Diel expression of cell cycle-related genes in synchronized cultures of Prochlorococcus sp. strain PCC 9511. J Bacteriol 183:915–920

    Article  PubMed  CAS  Google Scholar 

  • Holtzendorff J, Marie D, Post AF, Partensky F, Rivlin A, Hess WR (2002) Synchronized expression of ftsZ in natural Prochlorococcus populations of the Red Sea. Environ Microbiol 4:644–653

    Article  PubMed  CAS  Google Scholar 

  • Holtzendorff J, Partensky F, Mella D, Lennon JF, Hess WR, Garczarek L (2008) Genome streamlining results in loss of robustness of the circadian clock in the marine cyanobacterium Prochlorococcus marinus PCC 9511. J Biol Rhythms 23:187–199

    Article  PubMed  CAS  Google Scholar 

  • Homma K, Hastings JW (1989) Cell growth kinetics, division asymmetry, and volume control at division in the marine dinoflagellate Gonyaulax polyedra: a model of circadian clock control of the cell cycle. J Cell Science 92:303–318

    PubMed  Google Scholar 

  • Huang T-C, Tu J, Chow T-J, Chen T-H (1990) Circadian rhythm of the prokaryote Synechococcus sp. RF-1. Plant Physiol 92:531–533

    Article  PubMed  CAS  Google Scholar 

  • Ishii N, Nakahigashi K, Baba T, Robert M, Soga T, Kanai A, Hirasawa T, Naba M, Hirai K, Hoque A, Ho P Y, Kakazu Y, Sugawara K, Igarashi S, Harada S, Masuda T, Sugiyama N, Togashi T, Hasegawa M, Takai Y, Yugi K, Arakawa K, Iwata N, Toya Y, Nakayama Y, Nishioka T, Shimizu K, Mori H, Tomita M (2007) Multiple high-throughput analyses monitor the response of E. coli to perturbations. Science 316:593–597

    Article  PubMed  CAS  Google Scholar 

  • Ishiura M, Kutsuna S, Aoki S, Iwasaki H, Andersson CR, Tanabe A, Golden SS, Johnson CH, Kondo T (1998) Expression of a gene cluster kaiABC as a circadian feedback process in cyano-bacteria. Science 281:1519–1523

    Article  PubMed  CAS  Google Scholar 

  • Jacquet S, Partensky F, Marie D, Casotti R, Vaulot D (2001) Cell cycle regulation by light in Prochlorococcus strains. Appl Environ Microbiol 67:782–790

    Article  PubMed  CAS  Google Scholar 

  • Jarrett RM, Edmunds LN Jr (1970) Persisting circadian rhythm of cell division in a photosynthetic mutant of Euglena. Science. 167:1730–1733

    Article  PubMed  CAS  Google Scholar 

  • Johnson CH, Golden SS, Ishiura M, Kondo T (1996) Circadian clocks in prokaryotes. Mol Microbiol 21:5–11

    Article  PubMed  CAS  Google Scholar 

  • Kitayama Y, Iwasaki H, Nishiwaki T, Kondo T (2003) KaiB functions as an attenuator of KaiC phosphorylation in the cyanobacterial circadian clock system. EMBO J 22:2127–2134

    Article  PubMed  CAS  Google Scholar 

  • Koksharova OA, Wolk CP (2002) A novel gene that bears a DnaJ motif influences cyanobacterial cell division. J Bacteriol 184:5524–5528

    Article  PubMed  CAS  Google Scholar 

  • Kondo T, Strayer CA, Kulkarni RD, Taylor W, Ishiura M, Golden SS, Johnson CH (1993) Circadian rhythms in prokaryotes: luciferase as a reporter of circadian gene expression in cyanobacteria. Proc Natl Acad Sci USA 90:5672–5676

    Article  PubMed  CAS  Google Scholar 

  • Kondo T, Tsinoremas NF, Golden SS, Johnson CH, Kutsuna S, Ishiura M (1994) Circadian clock mutants of cyanobacteria. Science 266:1233–1236

    Article  PubMed  CAS  Google Scholar 

  • Kondo T, Mori T, Lebedeva NV, Aoki S, Ishiura M, Golden SS (1997) Circadian rhythms in rapidly dividing cyanobacteria. Science 275:224–227

    Article  PubMed  CAS  Google Scholar 

  • Laub MT, McAdams HH, Feldblyum T, Fraser CM, Shapiro L (2000) Global analysis of the genetic network controlling a bacterial cell cycle. Science 290:2144–2148

    Article  PubMed  CAS  Google Scholar 

  • Lee D-Y, Rhee G-Y (1999) Circadian rhythm in growth and death of Anabaena flosaquae (cyano-bacteria). J Phycol 35:694–699

    Article  Google Scholar 

  • Lewis PJ (2004) Bacterial subcellular architecture: recent advances and future prospects. Mol Microbiol 54:1135–1150

    Article  PubMed  CAS  Google Scholar 

  • Liu Y, Golden SS, Kondo T, Ishiura M, Johnson CH (1995) Bacterial luciferase as a reporter of circadian gene expression in cyanobacteria. J Bacteriol 177:2080–2086

    PubMed  CAS  Google Scholar 

  • Liu Y, Tsinoremas NF, Johnson CH, Lebedeva NV, Golden SS, Ishiura M, Kondo T (1996) Circadian orchestration of gene expression in cyanobacteria. Genes Dev 9:1469–1478

    Article  Google Scholar 

  • Löwe J, van den Ent F, Amos LA (2004) Molecules of the bacterial cytoskeleton. Annu Rev Biophys Biomol Struct 33:177–198

    Article  PubMed  CAS  Google Scholar 

  • Mackey SR, Choi JS, Kitayama Y, Iwasaki H, Dong G, Golden SS (2008) Proteins found in a CikA-interaction assay link the circadian clock, metabolism, and cell division in Synechococcus elongatus. J Bacteriol 190:3738–3746

    Article  PubMed  CAS  Google Scholar 

  • McAdams HH, Shapiro L (2003) A bacterial cell-cycle regulatory network operating in time and space. Science 301:1874–1877

    Article  PubMed  CAS  Google Scholar 

  • Matsuo T, Yamaguchi S, Mitsui S, Emi A, Shimoda F, Okamura H (2003) Control mechanism of the circadian clock for timing of cell division in vivo. Science 302:255–259

    Article  PubMed  CAS  Google Scholar 

  • Mihalcescu I, Hsing W, Leibler S (2004) Resilient circadian oscillator revealed in individual cyanobacteria. Nature 430:81–85

    Article  PubMed  CAS  Google Scholar 

  • Miller BH, McDearmon EL, Panda S, Hayes KR, Zhang J, Andrews JL, Antoch MP, Walker JR, Esser KA, Hogenesch JB, Takahashi JS (2007) Circadian and CLOCK-controlled regulation of the mouse transcriptome and cell proliferation. Proc Natl Acad Sci USA 104:3342–3347

    Article  PubMed  CAS  Google Scholar 

  • Min H, Guo H, Xiong J (2005) Rhythmic gene expression in a purple photosynthetic bacterium, Rhodobacter sphaeroides. FEBS Lett 579:808–812

    Article  PubMed  CAS  Google Scholar 

  • Mitsui A, Kumazawa S, Takahashi A, Ikemoto H, Cao S, Arai T (1986) Strategy by which nitrogen-fixing unicellular cyanobacteria grow photoautotrophically. Nature 323:720–722

    Article  CAS  Google Scholar 

  • Miyagishima S Y, Wolk CP, Osteryoung KW (2005) Identification of cyanobacterial cell division genes by comparative and mutational analyses. Mol Microbiol 56:126–143

    Article  PubMed  CAS  Google Scholar 

  • Mori T, Johnson CH (2000) Circadian control of cell division in unicellular organisms. Prog Cell Cycle Res 4:185–192

    PubMed  CAS  Google Scholar 

  • Mori T, Johnson CH (2001) Independence of circadian timing from cell division in cyanobacteria. J Bacteriol 183:2439–2444

    Article  PubMed  CAS  Google Scholar 

  • Mori T, Binder B, Johnson CH (1996) Circadian gating of cell division in cyanobacteria growing with average doubling times of less than 24 hours. Proc Natl Acad Sci USA 93:10183–10188

    Article  PubMed  CAS  Google Scholar 

  • Moulager M, Monnier A, Jesson B, Bouvet R, Mosser J, Schwartz C, Garnier L, Corellou F, Bouget FY (2007) Light-dependent regulation of cell division in Ostreococcus: evidence for a major transcriptional input. Plant Physiol 144:1360–1369

    Article  PubMed  CAS  Google Scholar 

  • Nagoshi E, Saini C, Bauer C, Laroche T, Naef F, Schibler U (2004) Circadian gene expression in individual fibroblasts: cell-autonomous and self-sustained oscillators pass time to daughter cells. Cell 119:693–705

    Article  PubMed  CAS  Google Scholar 

  • Nakajima M, Imai K, Ito H, Nishiwaki T, Murayama Y, Iwasaki H, Oyama T, Kondo T (2005) Reconstitution of circadian oscillation of cyanobacterial KaiC phosphorylation in vitro. Science 308:414–415

    Article  PubMed  CAS  Google Scholar 

  • Nikaido SS, Johnson CH (2000) Daily and circadian variation in survival from ultraviolet radiation in Chlamydomonas reinhardtii. Photochem Photobiol 71:758–765

    Article  PubMed  CAS  Google Scholar 

  • Paietta JV (1982) Photooxidation and the evolution of circadian rhythmicity. J Theor Biol 97:77–82

    Article  PubMed  CAS  Google Scholar 

  • Pittendrigh CS (1965) Biological clocks, the functions, ancient and modern, of biological oscillations. In: Air Force Office of Scientific Research (ed) Science and the sixties, the proceedings of the cloudcroft symposium. Air Force Office of Scientific Research, Arlington, pp 96–111

    Google Scholar 

  • Pittendrigh CS (1993) Temporal organization: reflections of a Darwinian clock-watcher. Annu Rev Physiol 55:17–54

    Article  Google Scholar 

  • Quardokus EM, Brun YV (2003) Cell cycle timing and developmental checkpoints in Caulobacter crescentus. Curr Opin Microbiol 6:541–549

    Article  PubMed  CAS  Google Scholar 

  • Rogers LA, Greenbank GR (1930) The intermittent growth of bacterial cultures. J Bacteriol 19:181–190

    PubMed  CAS  Google Scholar 

  • Romberg L, Levin PA (2003) Assembly dynamics of the bacterial cell division protein FtsZ: poised at the edge of stability. Annu Rev Microbiol 57:125–154

    Article  PubMed  CAS  Google Scholar 

  • Rothfield L, Justice S, García-Lara J (1999) Bacterial cell division. Annu Rev Genet 33:423–448

    Article  PubMed  CAS  Google Scholar 

  • Scheving LE (1981) Circadian rhythms in cell proliferation: their importance when investigating the basic mechanism of normal versus abnormal growth. Prog Clin Biol Res 59C:39–79

    PubMed  CAS  Google Scholar 

  • Schmitz O, Katayama M, Williams SB, Kondo T, Golden SS (2000) CikA, a bacteriophytochrome that resets the cyanobacterial circadian clock. Science 289:765–768

    Article  PubMed  CAS  Google Scholar 

  • Shih YL, Rothfield L (2006) The bacterial cytoskeleton. Microbiol Mol Biol Rev 70:729–754

    Article  PubMed  CAS  Google Scholar 

  • Siede W, Friedberg EC (1990) Influence of DNA repair deficiencies on the UV sensitivity of yeast cells in different cell cycle stages. Mutat Res 245:287–292

    Article  PubMed  CAS  Google Scholar 

  • Sinhaa RP, Häderb DP (2008) UV-protectants in cyanobacteria. Plant Sci 174:278–289

    Google Scholar 

  • Smith RM, Williams SB (2006) Circadian rhythms in gene transcription imparted by chromosome compaction in the cyanobacterium Synechococcus elongatus. Proc Natl Acad Sci USA 103:8564–8569

    Article  PubMed  CAS  Google Scholar 

  • Sturtevant R (1973a) Circadian patterns in linear growth of Escherichia coli. Anat Rec 175:453

    Google Scholar 

  • Sturtevant R (1973b) Circadian variability in Klebsiella demonstrated by cosinor analysis. Int J Chronobiol 1:141–146

    CAS  Google Scholar 

  • Sweeney BM, Borgese MB (1989) A circadian rhythm in cell division in a prokaryote, the cyano-bacterium Synechococcus WH7803. J Phycol 25:183–186

    Article  Google Scholar 

  • Sweeney BM, Hastings JW (1958) Rhythmic cell division in populations of Gonyaulax polyedra. J Protozool 5:217–224

    Google Scholar 

  • Vicente M, Rico AI, Martínez-Arteaga R, Mingorance J (2006) Septum enlightenment: assembly of bacterial division proteins. J Bacteriol 188:19–27

    Article  PubMed  CAS  Google Scholar 

  • Ward JE Jr, Lutkenhaus J (1985) Overproduction of FtsZ induces minicell formation in E. coli. Cell 42:941–949

    Article  PubMed  CAS  Google Scholar 

  • Wille JJ Jr, Ehret CF (1968) Light synchronization of an endogenous circadian rhythm of cell division in Tetrahymena. J Protozool 15:785–789

    PubMed  Google Scholar 

  • Woelfle MA, Xu Y, Qin X, Johnson CH (2007) Circadian rhythms of superhelical status of DNA in cyanobacteria. Proc Natl Acad Sci USA 104:18819–18824

    Article  PubMed  CAS  Google Scholar 

  • Zhang X, Dong G, Golden SS (2006) The pseudo-receiver domain of CikA regulates the cyano-bacterial circadian input pathway. Mol Microbiol 60:658–668

    Article  PubMed  CAS  Google Scholar 

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Mori, T. (2009). Cell Division Cycles and Circadian Rhythms. In: Ditty, J.L., Mackey, S.R., Johnson, C.H. (eds) Bacterial Circadian Programs. Springer, Berlin, Heidelberg. https://doi.org/10.1007/978-3-540-88431-6_11

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