LAUSR

KaiC is the central cog of the circadian clock in Cyanobacteria. Close homologs of this protein are widespread among nonphotosynthetic bacteria, but the function, interaction network, and mechanism of action of these proteins are still largely unknown. Here, we focus on KaiC homologs found in environmental Pseudomonas species. Using bioinformatics, we describe the distribution of this protein family in the genus and reveal a conserved interaction network comprising a histidine kinase and response regulator. We characterize experimentally the only KaiC homolog present in Pseudomonas putida KT2440 and Pseudomonas protegens CHA0. Through phenotypic assays and transcriptomics, we show that KaiC is involved in osmotic and oxidative stress resistance in P. putida and in biofilm production in both species. KaiC homologs are found in different phosphorylation states and physically interact with a cognate histidine kinase and response regulator. In contrast with cyanobacterial counterparts, the expression and phosphorylation of KaiC homologs do not correlate with light variations under 12:12 light:dark cycles in either Pseudomonas species, and KaiC itself is not required to support a light-driven behavior in P. putida. Overall, this suggests that KaiC homologs in Pseudomonas species are involved in environmental stress resistance, but not specifically in responses to diurnal rhythms. This article is protected by copyright. All rights reserved.