FIG. 1.

Classification scheme for two-component system genes. Schematic diagram of classification schemes for two-component system genes. The definition of paired and orphan TCS genes includes information about transcription direction as indicated by the arrow symbols. Complex TCS gene clusters include clusters containing two or more RR genes, clusters containing two or more HPK or HPK-like genes, and clusters with three or more TCS genes irrespective of transcription direction, as indicated by the box symbols. For complex gene clusters, only the most common gene organizations are shown (see Tables S3, S4, and S5 in the supplemental material for all gene organizations found in these clusters).