Much is now known about chemotaxis signaling transduction for Escherichia coli and Salmonella typhimurium. The mechanism of chemotaxis of Bacillus subtilis is, in a sense, reversed. Attractant binding strengthens the activity of histidine kinase in B. subtilis, instead of an inhibition reaction. The HemAT from B. subtilis can detect oxygen and transmit the signal to regulatory proteins that control the direction of flagella rotation. We have determined the crystal structures of the HemAT sensor domain in liganded and unliganded forms at 2.15 A and 2.7 A resolution, respectively. The liganded structure reveals a highly symmetrical organization. Tyrosine70 shows distinct conformational changes on one subunit when ligands are removed. Our study suggests that disruption of the symmetry of HemAT plays an important role in initiating the chemotaxis signaling transduction cascade.