E. coli has two oxygen sensors: FNR (fumarate-nitrate reductase) and ArcBA (Anoxic Redox Control, two component control systems). FNR directly senses the oxygen, while the interaction of ArcB with oxygen is indirect one via quinone pool. Out of theree quionones in E. coli, which one (oxidized and reduced forms of all three types) play roles to what extent in what direction (inhibiting/activating) is much debatable. I would like to see some intuitive level arguments for why a cell, even in principle, need to have two sensors for oxygen?
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It is not uncommon for cells to have parallel pathways for same outcome. This ensures foolproof response and makes the system robust. E.coli also has another sensor for aerotaxis (Aer and Tsr proteins). See my answer on your previous post and the linked paper.
Also look for coherent feed forward network motifs.
