Model of merodiploid formation during pneumococcal transformation.

<p>(A) Alternative pairing between R-NRf sequences triggers unequal crossing over. Letters as in <a href="http://www.plosgenetics.org/article/info:doi/10.1371/journal.pgen.1003819#pgen-1003819-g001" target="_blank">Figure 1D</a>. (B) Spontaneous restoration of complementary strand integrity. (C) Restoration of complementary strand integrity stimulated by R₂-Z donor fragment. (D) Resolution of chromosome dimer to produce one tandem-duplicated chromosome and one abortive chromosome. <i>oriC</i>, origin of replication; <i>terC</i>, terminus of replication; Δ, deletion; †, abortive chromosome. (E) Predicted abortive chromosome structure. Layout as in <a href="http://www.plosgenetics.org/article/info:doi/10.1371/journal.pgen.1003819#pgen-1003819-g002" target="_blank">Figure 2A</a>. Red square; abortive chromosome junction. Detection of abortive chromosome junction by primers in <a href="http://www.plosgenetics.org/article/info:doi/10.1371/journal.pgen.1003819#pgen-1003819-g003" target="_blank">Figure 3C</a>. Diagram of R_1/2 sequence at abortive chromosome junction amplified after R₁-A transformation, determined by sequencing of PCR fragments. Vertical lines, R₁/R₂ mismatches; red horizontal line, mixed sequence .(F) Saturation of the Hex system establishes frequent alternative pairing of R-NRf fragment. Transformation efficiencies of Rif^R and Nov^R point mutations (carried by R304 chromosomal DNA) compared in <i>hex⁻</i> (R246) or <i>hex⁺</i> (R800) recipients in the presence or absence of R-NRf PCR fragments. Transformation efficiencies normalized to Sm^R (<i>rpsL41</i>) point mutation, resistant to Hex. <i>hex</i>⁻ recipient, shades of gray; <i>hex</i>⁺ recipient, shades of blue.