CIRCADIAN RHYTHMS - Cyanobacteria - page 326

measured the rhythms exhibited by the individual cells when seeded on agar plates and followed
them up after every cell division by capturing their images division after division. Such individual
cells of S . elongatus PCC 7942 exhibited robust and resilient circadian rhythms by the expression
of the luciferase gene. The perturbations caused due to high intracellular noise and frequent cell
divisions did not affect the individual rhythms and the cell to cell interactions did not exist. This
confi rms that the circadian clock of individual cyanobacterial cells is quite stable and there is no
intercellular coupling effect (Amdaoud et al ., 2007).
iii) Adaptive fi tness : It is generally accepted that circadian rhythms help the organism for better
adaptation to the environmental conditions but convincing tests to prove this point have come up
only in case of the cyanobacterium S . elongatus PCC 7942 (Kondo et a l., 1994; Ouyang et a l., 1998;
Ishiura et al ., 1998). Wild-type and mutant strains of this organism that exhibited differences in the
circadian period were grown in single strain and in mixed cultures. For example, mutant strains
C22a and C28a showed periods of 23 h and 30 h, respectively whereas wild-type had a period of
25 h. There was no difference in the performance of the mutant strains in LL and in LD cycles when
grown in single strain cultures suggesting that the difference in circadian periods did not contribute
to any added advantage or disadvantage to the respective mutant strain. When mutant and wild-
type strains were allowed to compete with each other in mixed cultures in different LD regimes
such as 22 h cycle (LD 11:11h), a 24 h cycle (LD 12:12 h) and 30 h cycle (LD 15:15 h), the particular
mutant whose period most closely matched that of LD-cycle emerged out successfully and eliminated
other strains including the wild-type (Fig. 6). Woelfe et al . (2004) emphasized the adaptive value
of circadian clocks in cyclic environments alone. A comparison of clock-disrupted cyanobacterial
strains with those having a functional biological clock revealed that the latter defeated the former
in rhythmic environments (Fig. 7). But this inherent advantage of the functional clock seems to
disappear in constant environments.
VI. KAI GENES IN OTHER CYANOBACTERIA
Apart from S . elongatus PCC 7942, kai genes have now been identifi ed in more than forty cyanobacteria
belonging to diverse taxonomic groups (Lorne et al ., 2000). In all the six cyanobacteria ( S. elongatus
PCC 7942, Synechocystis sp. strain PCC 6803, Anabaena sp. strain PCC 7120, Prochlorococcus marinus
MED4, P. marinus MIT 9313, Synechococcus sp. strain WH 8102 and N. punctiforme ATCC 29133) in
which these genes have now been sequenced, the ORFs resemble only two kai genes, i.e. kaiB and
kaiC . In addition, kaiA gene sequence is diversifi ed. Furthermore, the genome of Synechocystis sp.
strain PCC 6803 contains multiple gene sequences of kaiB and kaiC but it possesses only one kaiA gene.
Dvornyk et al . (2002) identifi ed a genetic polymorphism in kaiABC gene family in Nostoc muscorum
permanently exposed to harmful UV radiations at two sites (Lower Nahal Oven, Mount Carmel and
Lower Nahal Keziv, western upper Galilee) in Israel designated as “Evolution Canyons” (EC I and
II). Five distinct subfamilies, kai I to kai V consisting of 20 functional genes and pseudogenes have
been identifi ed with variation in the number of member genes in each subfamily. Subfamilies I and
II had 5 member genes each with 2 member genes in subfamilies IV and V while subfamily III had
6 member genes. Subfamilies IV and V occurred at EC I and EC II, respectively. Exceptionally, kai IV
subfamily genes were found only as a single copy halotypes whereas genes of the kai V subfamily
were observed in multiple copy halotypes. This polymorphism appeared to be evolutionarily recent as
such radiation of genes was absent in other closely related genera such as Nodularia , Cylindrospermum
and N. punctiforme ATCC 29133.