FIGURE 1.

A, model of the subunits of the archaellum. Sulfolobus sp. have a single cell membrane that is surrounded with a proteinaceous layer called S-layer. So far it is known that PibD, a dedicated type IV prepilin signal peptidase, processes FlaB, the sole archaellin, prior to assembly. FlaI, the only characterized archaella subunit that forms an ATP-dependent hexamer, probably interacts with the sole polytopic membrane protein FlaJ. FlaH is a predicted ATP-binding protein but its role in archaella biosynthesis or rotation is unknown. The localization and function of FlaX, FlaF, and FlaG the other three conserved archaellum proteins in Sulfolobus sp. is not known. CM, cytoplasmic membrane. B, the operon structure of archaellum. Arrowhead showing the promoters involved in transcription mRNA of archaella-specific proteins.