Research ArticleBIOENGINEERING

Transplantability of a circadian clock to a noncircadian organism

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Science Advances  12 Jun 2015:
Vol. 1, no. 5, e1500358
DOI: 10.1126/sciadv.1500358
  • Fig. 1 Functionality of the core KaiABC oscillator in a heterologous system.

    (A) Model for the phosphorylation state changes of KaiC through the antagonistic activities of KaiA and KaiB. (B) Levels of phosphorylated (-P) and unphosphorylated wild-type (WT) or nonphosphorylatable mutant (S431A, T432A; “KaiC-unP mut”) of KaiC-FLAG protein when expressed with KaiAB. Proteins were visualized by SDS–polyacrylamide gel electrophoresis (SDS-PAGE) and immunoblotting (see Materials and Methods). The ratio of phosphorylated KaiC to total KaiC is plotted. (C) Levels of phosphorylated and unphosphorylated KaiC-FLAG, coexpressed with KaiB, with or without KaiA overexpression (see Materials and Methods). The ratio of phosphorylated KaiC to total KaiC is plotted. The asterisk indicates higher proportion of phosphorylated KaiC when KaiA is overexpressed (P < 0.05, Student’s t test). (D) KaiC phosphorylation over time in E. coli coexpressing KaiA and KaiB, after synchronization (t = 0 hours). The mean ratio of phosphorylated KaiC to total KaiC across biological replicates, mean-normalized for each time trace, is plotted. Circadian oscillations are statistically significant as analyzed using RAIN (P < 0.001) (7). (E) Power of circadian periodicity of KaiABC clock strains (red) and control strains expressing only KaiC (blue). Traces produced by bandpass filtering for circadian periods (20 to 30 hours) followed by Fourier transform of time course data from biological replicates are plotted (n = 3). Vertical line indicates frequency corresponding to a 24-hour period. The asterisk indicates increased power of circadian periodicity in KaiABC clock strains (P < 0.05, Student’s t test). Error bars, SEM (n = 3) (B to D).

  • Fig. 2 A synthetic transcriptional reporter of KaiC phosphorylation demonstrates circadian oscillations.

    (A) Schematic of the synthetic transcriptional reporter of KaiC phosphorylation state. Expression of the green fluorescent protein (GFP) reporter is dependent on the interaction between KaiC, which is fused to the α subunit of RNA polymerase N-terminal-domain (KaiC-αNTD), and SasA, which is fused to λCI binding protein (SasA-CI). Inset: Thickness of arrows indicates differing binding strength of phosphorylated and unphosphorylated KaiC to SasA. (B) OD-normalized fluorescent reporter output from strains with either binding partner alone or both KaiC-αNTD and SasA-CI. Error bars, SEM (n = 3). (C) OD-normalized fluorescent reporter output of interactions between phosphomimic KaiC (S431A, T432E; -P mut), wild-type KaiC (WT), or a nonphosphorylatable mutant of KaiC (S431A, T432A; -unP mut) with WT SasA-CI. Error bars, SEM (n = 3). (D to G) Background-subtracted and OD-normalized time course fluorescence when coexpressing KaiC (D) or KaiC S431A T432A nonphosphorylatable mutant (F) with SasA fusion proteins and KaiAB. Brackets indicate 24-hour periods. Statistically significant circadian oscillations were observed in (D) using RAIN (P < 0.05). Maximum power, shown by Fourier analysis of (D) and (F), occurs at a period of ~24 hours in (E) and at periods of >30 and/or <20 hours in controls (G). Dashed line indicates a 24-hour period. t = 0 indicates synchronization. (H) Background fluorescence, which was subtracted in (D) and (F), from a strain containing only the reporter and no clock components.

  • Fig. 3 Single cells demonstrate oscillatory behavior.

    (A) Fluorescent cells in channels in the microfluidic device (orange outline) with medium flowing (gray arrows) across the base of the channels, allowing for long-term microscopy assay of single-cell fluorescence. (B) Fluorescent cells in one channel of the microfluidic device at one time point (left). Kymograph shows fluorescence of a single channel over time (right), after overnight induction and minimal medium shock synchronization (at t = 0). Time interval, 1 hour. (C) Average fluorescence of a single mother cell, grown as in (A) and (B), containing the synthetic reporter and Kai clock components. (D) Bandpass-filtered (circadian periods of 20 to 30 hours) data from (B) to compare the strength of circadian periodicity across multiple single-cell traces. (E) Fourier spectra of the time courses of Kai clock strains (red, N = 65) or control strains containing only the reporter (blue, N = 48), both filtered as in (D). Eleven percent of cells had a higher power than the control cell with the highest power. Asterisk indicates that the overall distribution of power of the circadian clock cells is greater than that of controls (one-tailed, two-sample Kolmogorov-Smirnov test, P = 0.001). Black line indicates frequency corresponding to a 24-hour period.

  • Fig. 4 KaiC transmits phosphorylation information to cyanobacterial circadian promoters in E. coli.

    (A) Reporter fluorescence driven by cyanobacterial circadian-responsive promoters (kaiBC, rpoD6, or sigF2), with or without the Kai clock components. (B) Reporter fluorescence driven by E. coli housekeeping gene promoter, ihfB, with or without Kai clock components. All measurements were taken after overnight induction. Error bars, SEM (n = 3).

Supplementary Materials

  • Supplementary material for this article is available at http://advances.sciencemag.org/cgi/content/full/1/5/e1500358/DC1

    Fig. S1. Plasmids built for reconstruction of the circadian oscillator in E. coli.

    Fig. S2. Additional data and quantifications of KaiC phosphorylation in E. coli expressing KaiABC.

    Fig. S3. Circadian phosphorylation of KaiC over time requires KaiA and KaiB.

    Fig. S4. Plasmids built for the synthetic oscillator utilizing a modified bacterial two-hybrid system.

    Fig. S5. KaiC and SasA phosphorylation states affect reporter output.

    Table S1. Raw data for Fig. 2D.

    Table S2. Bacterial strains and plasmids.

    Movie S1. Circadian oscillations visualized in single E. coli cells using a microfluidic device.

  • Supplementary Materials

    This PDF file includes:

    • Fig. S1. Plasmids built for reconstruction of the circadian oscillator in E. coli.
    • Fig. S2. Additional data and quantifications of KaiC phosphorylation in E. coli expressing KaiABC.
    • Fig. S3. Circadian phosphorylation of KaiC over time requires KaiA and KaiB.
    • Fig. S4. Plasmids built for the synthetic oscillator utilizing a modified bacterial two-hybrid system.
    • Fig. S5. KaiC and SasA phosphorylation states affect reporter output.
    • Table S1. Raw data for Fig. 2D.
    • Table S2. Bacterial strains and plasmids.

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    Other Supplementary Material for this manuscript includes the following:

    • Movie S1 (.mov format). Circadian oscillations visualized in single E. coli cells using a microfluidic device.

    Files in this Data Supplement: