CIRCADIAN RHYTHMS - Cyanobacteria - page 325

glucose in darkness when the cells are exposed to daily brief light pulses. This has been termed as
light-activated heterotrophic growth. With the help of dnaK::luxAB reporter strain of Synechocystis ,
it was demonstrated that the luminescence rhythm persisted for many cycles in DD. Likewise,
Schneegurt et al . (1997) demonstrated that in Cyanothece sp. ATCC 51142 the rhythms of nitrogenase
activity and carbohydrate content persisted for 4 days when grown on glycerol. These studies thus
supported the idea that the circadian clock can run in DD if the specifi c metabolic rate is maintained
and that the clock is not dependent on light. To sum up, it can be stated that the photoautotrophic
cyanobacteria continue to operate the circadian clock even in DD as also the heterotrophic
cyanobacteria that exhibit the circadian rhythms in DD. It means as light serves as a signal in the
input pathway, so also DD serves as a signal for the circadian clock.
Cervený and Nedbal (2009) established a close relationship between respiratory activity and
glycogen degradation in the long day (LD cycle of 16:8) condition coinciding with a transition from
aerobic photosynthetic metabolism and dark respiratory phase at the dusk period. They extrapolated
these fi ndings to the accumulation of KaiB tetramer formation and the apparent link with circadian
mechanism and the dark respiration in Cyanothece sp. ATCC 51142.
ii) Cell division and the rhythms : Early studies indicated that there existed a bi-directional
interdependent linkage between circadian rhythms and cell division cycle (CDC) (Ehet and Wille,
1970; Klevecz, 1976; Edmunds, 1988). This is especially true for unicellular eukaryotic organisms
where the timing of CDC is determined by circadian oscillator. It means that it is the circadian
oscillator that specifi es the time for cell division to occur even though the cells have attained the
maturity to undergo the division process. An alternative suggested is that circadian clock mechanism
oscillates independently of CDC but the CDC is dependent on the phase of circadian clock in a manner
that is gated to occur coinciding with circadian phases (Ehet and Wille, 1970; Edmunds, 1988; Goto
and Johnson, 1995). Mori et al . (1996) examined the rates of cell division and DNA content in rapidly
growing cells of S . elongatus PCC 7942 and its mutant strain AMC149 that had psbAI :: luxAB reporter
gene set in its genome and two other mutant strains C22a and C27a (previously designated as SP22
and LP27, respectively, isolated after chemical mutagenesis of AMC149). Cells dividing at rates as
fast as 10 h were controlled by circadian oscillator for cell division and gene expression as well as
DNA replication occurred continuously throughout the circadian cycle within the population of
dividing cells but the cytokinesis is gated by the circadian clock such that division is forbidden in
the early subjective night. So it was felt necessary to examine whether genes related to cytokinesis
(or septum formation) play a role in the circadian gating of cell division. A gene designated as ftsZ
that is ubiquitously present in bacteria and chloroplasts encodes FtsZ protein. This protein assembles
in the form of a Z ring in the middle of the cell that helps in the formation of a septum (Corton et
al ., 1987; Bi and Lutkenhaus, 1991). An ftsZ gene identifi ed from Anabaena was amplifi ed by PCR
and the PCR product was sequenced. Due to its similarity to regions of known ftsZ genes, it was
introduced with its promoter along with a bacterial luciferase gene set into S . elongatus PCC 7942.
It was found that the activity of ftsZ was highest when cells in the population were not dividing.
Overexpression of ftsZ halted cell division and caused cells to become fi lamentous. In such mutants,
robust circadian fl uctuations of other genes such as psbA1 , KaiBC and ftsZ persisted for at least 4
to 5 days that suggested regulation of gene expression and gating of cell division is not affected by
halting cell division (Mori et al ., 1996).
In the colonies of unicellular cyanobacteria, when the products of cell division get separated and
move away from one another do the rhythms exhibited by certain cells infl uence other cells or not?
Mihalceseu et al . (2004) examined this question by inserting psbAI :: luxAB and psbAI :: luxCDE and