Chemotaxis-like signalling systems in P . carbinolicus. The cytoplasmic domain of each chemoreceptor (or MCP) is a coiled coil of amino acid residue heptads (blue, numbered), looped in the middle. The number of heptads and the positions of methylation sites (pink) are unique to each class of MCP. Methylation by CheR and demethylation by CheB and CheD modulate the ability of MCP to bind CheW (or CheV), which activates CheA. Phosphorylation by CheA and dephosphorylation by CheX cause CheY to inhibit or stimulate flagellar rotation and other processes. Twelve MCP of P. carbinolicus are of class 36H, in contrast with one per genome in G. metallireducens, G. bemidjiensis, G. daltonii and Geobacter uraniireducens, and none in G. sulfurreducens, G. lovleyi or P. propionicus. Most MCP of Geobacteraceae belong to classes 40H (nine to eighteen per genome), 40+24H (one to eight per genome), and 34H (two to eight per genome), but P. carbinolicus has only two of class 40H. P. carbinolicus, like its relatives except G. metallireducens, has one MCP gene of class 44H, predicted to be co-transcribed with a gene for outer membrane lipoprotein carrier/sorting protein LolA (Pcar_0032). These minor MCP appear to belong to rudimentary signalling systems that must rely on the methyltransferases and demethylases of the major MCP for fine-tuning.