- and motor components (arlI, arlJ, arlH). The locus furthermore encodes other accessory proteins (arlG, arlF, arlC, arlD, arlE and arlX). ArlX is only found in Crenarchaeota and ArlCDE (which can exist as individual proteins or as fusion proteins) in Euryarchaeotes. ArlX and ArlCDE are thought to have similar functions, and an unknown protein also is thought to fulfil the same function in Thaumarchaeota.

The archaellum operon used to be historically