A comprehensive set of 6TM AC anchors was extracted from 408 eukaryotic and 1456 bacterial proteomes and clustered in CLANS (HHsearch p-value cutoff 5E-4, attraction value 10, repulsion value 5); outliers were removed. Each dot represents a single 6TM domain. Above threshold HHsearch hits are shown as connecting lines between AC pairs in different clusters; the darker line color, the more similar the protein sequences. 6TM anchors of ACs form five clusters of high pairwise sequence similarity: cluster (1), anchors of bacterial class IIIb ACs characterized by the presence of a cytosolic ferredoxin domain (30 sequences from the α and β branches of proteobacteria). Cluster (2), anchors of bacterial class IIIb ACs characterized by a signal-transducing HAMP domain (118 sequences mainly from Actinobacteria, but also from α-proteobacteria, δ-proteobacteria, Chlorobia and Thermoleophilia). Cluster (3), anchors of bacterial class IIIb ACs similar to HAMP-associated anchor domains but which lack a HAMP domain (42 sequences mainly from α-proteobacteria). Cluster (4), anchors from bacterial class IIIa ACs prototypically represented by the mycobacterial AC Rv1625c (35 from many different phyla of bacteria, including Actinobacteria, Proteobacteria, Chlorophyta, Spirochaetes, and Bacteriodetes). The enlarged asterisk denotes the position of the mycobacterial AC Rv1625c. Cluster (5), anchors of the pseudoheterodimeric eukaryotic class IIIa ACs (TM1 677 sequences, TM2 653 sequences). CqsS Cluster (6), 6TM domains of sensory His-Kinases similar to CqsS (from Bacteriodetes, Chlorobia, α-, β-, and γ-proteobacteria). (B) Length comparisons of the transmembrane helices and loops of 6TM membrane anchors/receptors/sensors. The data sets from clusters 4, 5 and 6 from Figure 7A were used supplemented with 250 cytochrome b561 proteins. In case no S.E.M. is visible the size of the vertical bar is within the line thickness of the respective bar. ‘α’-Numbering denotes the consecutive TM helices starting from the N-terminus, 'out' and 'in’ denote sequential extra- and intracellular loop sequences. The two horizontal lines are at the 5 and 20 aa level.