Classes of intergenic SLSs. Based on the orientation of flanking CDSs, higher stability intergenic SLSs (dG < -10 KCal/mole) have been sorted into three categories, as indicated at the bottom (see Results). The width of each stacked bar denotes the fraction of SLSs belonging to the three categories. The thickness of the bars is proportional to the cumulative sizes of IGRs (lengths below 25000 bp are not to scale, but are represented by a minimal bar width). Lines above bars represent the intergenic space, split by vertical dashes in three segments respectively corresponding, left to right, to the cumulative lengths of IGRs flanked by unidirectionally, divergently and convergently transcribed CDSs. According to the parameters adopted, no conv-IGS was found in the genome of M. genitalium (see row 13). Only IGRs ranging from 29 to 500 bp were taken into account, since smaller regions can not contain the shortest detectable SLSs, and bigger ones might derive from inaccurate genome annotation. Bacterial genomes are numbered 1 through to 40 as in Table 1.