Fig. 1

The circular E. coli chromosome exhibits a toroidal donut-shape that can be visualized upon cell expansion. a Schematic of a E. coli genome. Two FROS markers are shown in red (Ori1 marked by lacO arrays, targed by LacI-mCherry) and cyan (Ter3 marked by tetO arrays, targeted by TetR-mCerulean). b Time-lapse fluorescence images of an E. coli (dnaC2(ts) allel) cell growing into a lemon shape at 40 °C under A22 treatment. Top panel, phase contrast image; bottom panel, overlay of Ori focus (red) and Ter focus (cyan) on a grey-scale deconvolved image of the chromosome labeled by HU-mYPet. Time is indicated in hours. c Fluorescence images showing two opened circular chromosomes captured by different methods. Left: WF, wide-field image, and DEC, deconvolved image of that WF. Right: WF and SIM, structured-illumination microscopy image of that WF image. d Donut-shape chromosome of E coli, as imaged in 3D-reconstructed SIM. Orange outlines the cell contour. e Similar donut-shape chromosome images are obtained for different DNA-binding fluorescent labels (HU-mYPet, Fis-mYpet, H-NS-mYPet, DAPI; all DEC images). f Fluorescence images of early (left) and late (right) stages of DNA replication of circular chromosomes. Bottom: cartoon illustrations; black strands indicate newly replicated DNA. g Fluorescent image of a donut-shape E. coli genome shown as a heat map. Indicated are the ridge of the bundle (green dashed line), the oriC and dif genomic loci near the origin and terminus of replication (red and blue dots respectively), and the bundle width (blue line). h Histogram of chromosome bundle lengths measured along the bundle ridge (cf. panel G). n = 269. i Histogram of the average chromosome bundle widths quantified as the full-width-at-half-maximum of the peak intensity across the donut chromosomes. n = 269. Scale bars in B/C/E/F, 2 μm. Scale bars in D/G, 1 μm