The histidine operon is ancient.
Price
Morgan N
MN
Alm
Eric J
EJ
Arkin
Adam P
AP
D006639:Histidine / Q000235:genetics*; D009876:Operon / Q000235:genetics*; D010802:Phylogeny; D020560:Proteobacteria / Q000235:genetics*; D013995:Time
D006639:Histidine