This domain is exclusively found in YycI proteins in the low GC content Gram positive species. These two domains share the same structural fold with domains two and three of YycH [2] Pfam:PF07435. Both, YycH and YycI are always found in pair on the chromosome, downstream of the essential histidine kinase YycG. Additionally, both proteins share a function in regulating the YycG kinase with which they appear to form a ternary complex. Lastly, the two proteins always contain an N-terminal transmembrane helix and are localised to the periplasmic space as shown by PhoA fusion studies.

1: YycH and YycI interact to regulate the essential YycFG two-component system in Bacillus subtilis. Szurmant H, Mohan MA, Imus PM, Hoch JA; J Bacteriol. 2007;189:3280-3289. PMID:17307850

2: The crystal structure of Bacillus subtilis YycI reveals a common fold for two members of an unusual class of sensor histidine kinase regulatory proteins. Santelli E, Liddington RC, Mohan MA, Hoch JA, Szurmant H; J Bacteriol. 2007;189:3290-3295. PMID:17307848