, 2005). To this day existence of a circadian clock has been demonstrated for Plants, Animals and, among the Prokarya, exclusively in Cyanobacteria. However, there is evidence for circadian rhythms also in other Bacteria (Min et al., 2005) and in Archaea (Edgar et al., 2012). One of the first circadian rhythms in a unicellular prokaryotic

organism was reported for cell division in the marine Synechococcus sp. strain WH 7803 ( Sweeney and Borgese, 1989). This astonishing observation contradicted a former hypothesis stating that intracellular compartments are absolutely necessary for circadian timing. In 1993 the Sunitinib nmr freshwater cyanobacterium Synechococcus elongatus PCC 7942 (hereafter S. elongatus) emerged as a prokaryotic model organism Nutlin-3 chemical structure for circadian research because it was amenable to genetic manipulations and

molecular tools were available for this species ( Golden et al., 1987, Golden, 1988 and Kondo et al., 1993). After 20 years of investigations the molecular mechanism underlying the functioning of the prokaryotic core clock is well understood, though many processes, especially those involved in input and output pathways in the cyanobacterial cell await further elucidation. The core oscillator of S. elongatus consists solely of three proteins, KaiA, KaiB

and KaiC ( Ishiura et al., 1998). KaiC is the core component of this unique post-translational oscillator. Due to inverse modulation by KaiA and KaiB it intrinsically phosphorylates and dephosphorylates, which leads to phosphorylation cycles that display a period of about 24 h ( Iwasaki et al., 2002, Kitayama et al., 2003, Nishiwaki et al., 2004 and Xu et filipin al., 2003). All three kai genes together are found in Cyanobacteria exclusively. Thus, the KaiABC system cannot represent a general prokaryotic clock mechanism. However, sequences similar to KaiC, sometimes in combination with KaiB, were identified also outside the cyanobacterial phylum, in Proteobacteria, Chloroflexi and Archaea ( Aoki and Onai, 2009 and Dvornyk et al., 2003). Regarding other cyanobacterial species and particularly marine Cyanobacteria, the knowledge about circadian rhythms is very limited. One of the reasons for the rare studies on clock systems in marine Cyanobacteria is founded mainly by the lack of effective genetic manipulation systems. Our purpose for this review is to compare the well-studied S. elongatus clock system with information we have on circadian rhythms in other, particularly marine Cyanobacteria.