Electron transfer subunit of the periplasmic nitrate reductase complex NapAB. Receives electrons from the membrane-anchored tetraheme c-type NapC protein and transfers these to NapA subunit, thus allowing electron flow between membrane and periplasm. Essential for periplasmic nitrate reduction with nitrate as the terminal electron acceptor. Component of the periplasmic nitrate reductase NapAB complex composed of NapA and NapB. Induced at high levels by nitrate under anoxic conditions in the dark. Induced by oxic conditions, even in the absence of nitrate. It is suggested that the extent of electron flux to nitrate or oxygen, under anoxic or oxic conditions, respectively, serves as the signal for induction. Binds 2 heme C groups per subunit. Decrease in nitrate reductase activity in deletion mutants and failure to grow in dark under anaerobic conditions. Belongs to the NapB family.