title: Genome streamlining results in loss of robustness of the circadian clock in the marine cyanobacterium Prochlorococcus marinus PCC 9511.
abstract: The core oscillator of the circadian clock in cyanobacteria consists of 3 proteins, KaiA, KaiB, and KaiC. All 3 have previously been shown to be essential for clock function. Accordingly, most cyanobacteria possess at least 1 copy of each kai gene. One exception is the marine genus Prochlorococcus, which we suggest here has suffered a stepwise deletion of the kaiA gene, together with significant genome streamlining. Nevertheless, natural Prochlorococcus populations and laboratory cultures are strongly synchronized by the alternation of day and night, displaying 24-h rhythms in DNA replication, with a temporal succession of G1, S, and G2-like cell cycle phases. Using quantitative real-time PCR, we show here that in Prochlorococcus marinus PCC 9511, the mRNA levels of the clock genes kaiB and kaiC, as well as a few other selected genes including psbA, also displayed marked diel variations when cultures were kept under a light-dark rhythm. However, both cell cycle and psbA gene expression rhythms damped very rapidly under continuous light. In the closely related Synechococcus sp. WH8102, which possesses all 3 kai genes, cell cycle rhythms persisted over several days, in agreement with established cyanobacterial models. These data indicate a correlation between the loss of kaiA and a loss of robustness in the endogenous oscillator of Prochlorococcus and raise questions about how a basic KaiBC system may function and through which mechanism the daily "lights-on" and "lights-off" signal could be mediated.
authors.affiliation: Université Pierre et Marie Curie (Paris 06) and Centre National de la Recherche Scientifique (UMR 7144), Station Biologique, France.
authors.forename: Julia
authors.initials: J
authors.lastname: Holtzendorff
authors.forename: Frédéric
authors.initials: F
authors.lastname: Partensky
authors.forename: Daniella
authors.initials: D
authors.lastname: Mella
authors.forename: Jean-François
authors.initials: JF
authors.lastname: Lennon
authors.forename: Wolfgang R
authors.initials: WR
authors.lastname: Hess
authors.forename: Laurence
authors.initials: L
authors.lastname: Garczarek
mesh_terms: D000595:Amino Acid Sequence; D001483:Base Sequence; D056926:CLOCK Proteins; D017931:DNA Primers; D005434:Flow Cytometry; D016680:Genome, Bacterial; D008969:Molecular Sequence Data; D009876:Operon; D016133:Polymerase Chain Reaction; D048929:Prochlorococcus / Q000235:genetics / Q000502:physiology*; D017386:Sequence Homology, Amino Acid; D015534:Trans-Activators / Q000737:chemistry / Q000235:genetics / Q000502:physiology*
chemical_list: D017931:DNA Primers; D015534:Trans-Activators; D056926:CLOCK Proteins