Coverage of prokaryotic genomes with nonconserved and conserved gene strings. Conserved gene strings were from information-density-based alignments with E < 0.1. The union of nonconserved and conserved gene strings was considered to comprise the set of potential operons (see text). Pairs of closely related genomes, namely C. trachomatis/C. pneumoniae, M. genitalium/M. pneumoniae,B. subtilis/C. acetobutilicum, and M. jannaschii/M. thermoautotrophicum, were disregarded for these calculations.