The crystal structure of Bacillus subtilis YycI reveals a common fold for two members of an unusual class of sensor histidine kinase regulatory proteins.

YycI and YycH are two membrane-anchored periplasmic proteins that regulate the essential Bacillus subtilis YycG histidine kinase through direct interaction. Here we present the crystal structure of YycI at a 2.9-A resolution. YycI forms a dimer, and remarkably the structure resembles that of the two C-terminal domains of YycH despite nearly undetectable sequence homology (10%) between the two proteins.