**2.3 FdRcaE**

*Fremyella diplosiphon* harbor photoreceptor RcaE\_GAF sector is homologous to SyCcaS\_GAF. Genetic studies on FdRcaE revealed that it is a red light receptor, involved in the expression of operon *cpc2* encode synthesis of phycocyanin, [17] FdRcaE domain structure GAF, PAS and His kinase (**Figure 1C**), are parallel to SyCcaS (**Figure 1B**). Though the GAF domain is analogous to FdRcaE\_ and SyCcaS, their light response is different in which the SyCcaS is a green light receptor. In *F. diplosiphon*, the green light has been used to activate genes for phycoerythrin posttranslational modification and its linker polypeptides through the second signaling pathway by CBCR [18, 19]. The modern genome sequencing project would reveal the genetic background of the whole complementary chromatic acclimation process.
