Fig. 1

Histograms of distances between inserts for (a) Shigella chromosome and (b) Shigella virulence plasmid. The median distance between inserts is indicated by the dotted line. A plurality of inserts are separated by only a single bp for both in both the chromosome and the plasmid. The insets in (a) and (b) indicate the slight but detectable biases in transposon insert location using a weight matrix motif. The reverse cumulative plots show the observed fractions of distances between inserts for the chromosome (c) and the plasmid (d). In blue, the observed frequencies are plotted. In black, the expected frequencies are plotted, given a geometric distribution (negative binomial with the number of successes set to one) of inter-insert distances (see main text). For both the chromosome and the plasmid, there are considerably more large regions uninterrupted by transposons than one would expect given the geometric null model, observed as a shift of the curve to the right