SLSs in natural genomes and comparable random sequences. SLSs found in natural bacterial genomes (filled circles) and random sequences of the same base composition (empty circles) are plotted against genome size in panel A. Filled and empty circles are connected for clarity. The same SLSs, normalized according to genome size, are plotted as a function of the GC content in panel B. The grey curve was obtained by determining the number of SLSs in a 1 Mbase random sequence of the indicated GC%. Each point is the average of 10 sequences, bars indicate standard deviations. Pre-filtering of SLSs was not performed. In both panels letters indicate bacterial species as listed in Table 1.