A Focus-less cell can form from any of the two ends of a chromosome linearized 3 kb from <i>dif</i>.

<p>(A) Schematic representation of the terminus region in a linear chromosome interrupted at position 1585. <i>terD</i> to <i>terB</i>, <i>dif</i>, the <i>parS</i> sites used for microscopy experiments, and the <i>hipA hipB</i> genes are shown. <u>(B) Schematic representation of non-heritable focus loss on linear chromosomes and micrographs showing examples of focus loss</u> during growth of <i>recB</i> cells in which the chromosome is linearized 3 kb from <i>dif</i> and carries either <i>yddW</i>:: <i>parS</i>_pMT1 (left panels) or <i>ydeV</i>:: <i>parS</i>_{pM T1} (right panels). Time-lapse experiments were carried out as in <a href="http://www.plosgenetics.org/article/info:doi/10.1371/journal.pgen.1007256#pgen.1007256.g004" target="_blank">Fig 4A</a>. Cells that generate a focus-less cell during division are circled with a full white line. Cells that have lost the focus are circled with a dashed white line. Loss of the <i>yddW</i>::<i>parS</i>_pMT1 focus occurring in image 5 (left panel) is not heritable, but focus-less cells divide because the <i>hipA hipB</i> genes are intact. Loss of the <i>ydeV</i>::<i>parS</i>_pMT1 focus occurring in image 17 (right panel) is not heritable, and focus-less cells do not divide because <i>hipB</i> is degraded. Another example of <i>ydeV</i>:: <i>parS</i>_pMT1 focus loss from a linear chromosomes is shown in <a href="http://www.plosgenetics.org/article/info:doi/10.1371/journal.pgen.1007256#pgen.1007256.s016" target="_blank">S6 Video</a> and the complete movie corresponding to the <i>yddW</i>:: <i>parS</i>_pMT1 images shown here is shown in <a href="http://www.plosgenetics.org/article/info:doi/10.1371/journal.pgen.1007256#pgen.1007256.s017" target="_blank">S7 Video</a>. <u>C. Ratio of normalized sequence reads in RecB+ over <i>recB</i> mutant cells with a linear chromosome</u>. Because <i>hipAB</i> is next to <i>dif</i>, cells that degrade this chromosome end do not multiply because they are blocked by the HipA toxin and become underrepresented in the population. Cells that degrade the other chromosome end multiply, which increases their relative amount in the population. Consequently, DNA loss in the population is amplified on the <i>yddW</i>::<i>parS</i>_pMT1 side and underestimated at the other end. Original normalized profiles used to calculate ratios are shown in <a href="http://www.plosgenetics.org/article/info:doi/10.1371/journal.pgen.1007256#pgen.1007256.s008" target="_blank">S4 Fig</a>. We observed that our linear strain carries a deletion of about 50 kb around positions 1400 to 1450, which was not observed previously and may be specific for our isolate.