Phylogenetic tree of HisA sequences. Congruency groupings that match TCG clusters are labelled 'HCG' for histidine congruency group. Helicobacter pylori and Streptococcus pneumoniae are not represented on the tree because they have lost the histidine pathway. C. jejuni HisA appears to be a xenologous member of TCG-1. Similar to the relatively loose TCG-2 and TCG-7 groupings, HCG-2 and HCG-8 are fragmented. Actinomycete bacteria possess a tightly clustered section of dual-pathway HisA (PriA) sequences within HCG-5.