New classes of mutants in complementary chromatic adaptation provide evidence for a novel four-step phosphorelay system.

Complementary chromatic adaptation appears to be controlled by a complex regulatory system with similarity to four-step phosphorelays. Such pathways utilize two histidine and two aspartate residues for signal transduction. Previous studies of the signaling system controlling complementary chromatic adaptation have uncovered two elements of this pathway, a putative sensor, RcaE, and a response regulator, RcaC. In this work, we describe a second response regulator controlling complementary chromatic adaptation, RcaF, and identify putative DNA binding and histidine phosphoacceptor domains within RcaC. RcaF is a small response regulator with similarity to SpoOF of Bacillus subtilis; the latter functions in the four-step phosphorelay system controlling sporulation. We have also determined that within this phosphorelay pathway, RcaE precedes RcaF, and RcaC is probably downstream of RcaE and RcaF. This signal transduction pathway is novel because it appears to use at least five, instead of four, phosphoacceptor domains in the phosphorelay circuit.