Gasbkm

Ability of the Synechococcus elongates PCC
7942 circadian clock under directed anti-phase
expression of the koi genes
Jayne L. Ditty,
1
4 Shannon R. Canales,
2
4 Breanne E. Anderson,
1
Stanly B. Williams
2
3 and Susan S. Golden
2
Correspondence
Susan S. Golden
sgolden@tamu.edu
1
Department of Biology, The University of St Thomas, St Paul, MN 55105, USA
2
Department of Biology,Texas A&M University, 3258 TAMU, College Station, TX 77843-3258,
USA
Received 11 March 2005
Revised 5 May 2005
Accepted 9 May 2005
The kayak, koi and koi genes encode the core components of the cyanobacteria circadian
Clock in Synechococcus elongates PCC 7942. Rhythmic expression patterns of kayak and of the
kick operon normally peak in synchrony. In some mutants the relative timing ofpeaks (phase
Relationship) between these transcription units is altered, but circadian rhythms persist robustly.
In this study, the importance of the transcriptional timing of kais genes was examined.
Expressing either kayak or laic from a heterologous promoter whose peak expression occurs
12 h out of phase from the norm, and thus 12 h out of phase from the other kais locus, did not affectthe time required for one cycle (period) or phase of the circadian rhythm, as measured by
bioluminescence reporters. Furthermore, the data confirm that specific cis elements within the
promoters of the kai genes are not necessary to sustain clock function.
INTRODUCTION
As a consequence of the Earth’s rotation about its axis once
every 24 h, virtually every organism that inhabits this planet
issubject to fluctuations in light and temperature. Organisms have evolved elaborate regulatory mechanisms, called
circadian clocks, to control daily behaviours based upon the
time of day, as an adaptation to the predictable periodicity
of environmental parameters. These daily rhythmic oscillations in behaviour, called circadian rhythms, are found in
organisms ranging from bacteria to humans, andpersist
under constant conditions; the time to complete one cycle
(circadian period) is temperature compensated, and the
period and relative timing of peaks (phase relationship) are
entrained to the sidereal day by environmental stimuli
(Ditty et al., 2003; Dunlap et al., 2004; Pittendrigh, 1981;
Young & Kay, 2001). In the cyanobacterium Synechococcus
elongatus PCC 7942, the prokaryoticmodel system for
circadian rhythms (Ditty et al., 2003; Golden & Canales,
2003), the circadian clock is a pervasive regulatory mechanism that controls most, if not all, gene expression in the
organism (Liu et al., 1995). Rhythmic transcription in S.
elongatus can be easily measured by following bioluminescence from the expression of promoters fused to luciferase
reporter genes (Anderssonet al., 2000; Canales et al., 2005).
In S. elongatus, the circadian pacemaker consists of the
products of at least three genes, kaiA, kaiB and kaiC. The kai
locus is expressed from two promoters – one upstream of
kaiA (PkaiA, monocistronic message) and one upstream of
kaiB (PkaiBC, dicistronic kaiBC message) – which drive
transcription in the same circadian phase in wild-type cells,
withpeak expression at the time that corresponds to dusk
when cells are kept in continuous light (subjective dusk)
(Ishiura et al., 1998). Central to models for the mechanism
of eukaryotic circadian clocks are delayed transcriptional–
translational feedback loops that regulate the timing of
clock-gene expression. These feedback loops involve
positive-effector proteins that stimulate coreclock-gene
expression. The core clock components then negatively
regulate their own expression, to generate an oscillation of
gene expression and clock-protein production that results
in a 24 h regulatory mechanism (Dunlap, 1999; Dunlap
et al., 2004; Harmer et al., 2001; Young & Kay, 2001). Some
aspects of the regulation of clock genes in S. elongatus are
reminiscent of autoregulatory...