Receptor Histidine Kinases in Green Algae: Domain Analysis

In bacteria and some eukaryotes, an important tool to respond to both external and internal signals is a two-component system, which is typically composed of a transmembrane receptor histidine kinase (RHK) and its cognate response regulator. While architecture and distribution of RHKs in prokaryotes are well-documented, their roles in eukaryotes, particularly in the green algae, remain less understood. Here, we analyzed the sequences and domain architectures of RHKs within the green algae clade Chlorophyta, in the class Chloropicophyceae, better characterized compared to other classes. Our study aims to deepen understanding and infer the biological roles of these proteins. Based on InterProScan results, we showed that rhodopsin-histidine kinases play a global role in the biology of the microalgae: they dominate among all RHKs, representing 57% of them. The length variation of RHKs does not have a discrete structure. Furthermore, we identified the most frequent domains and domain architectures in the proteins, with a notable discovery of a significant number of hybrid histidine kinases, where the sensory domain is a microbial rhodopsin protein. We demonstrate also that one-half of the identified rhodopsin-histidine kinases possess a catalytic domain of either adenylyl or guanylyl cyclase, making them potentially useful as optogenetic tools. In addition, it is shown that a significant part of RHKs in green algae contain not one but two REC-domains. These findings, along with previous reports, suggest that multiple REC-domains may support complex regulatory functions, possibly allowing condition-dependent responses. Our work may also help to understand better RHKs potential in optogenetics.