LuxR-type regulators play important roles in transcriptional regulation in 
bacteria and control various biological processes. A genome sequence analysis 
showed the existence of seven LuxR-type regulators in Azorhizobium caulinodans 
ORS571, an important nitrogen-fixing bacterium in both its free-living state and 
in symbiosis with its host, Sesbania rostrata. However, the functional 
mechanisms of these regulators remain unclear. In this study, we identified a 
LuxR-type regulator that contains a cheY-homologous receiver (REC) domain in its 
N terminus and designated it AclR1. Interestingly, phylogenetic analysis 
revealed that AclR1 exhibited relatively close evolutionary relationships with 
MalT/GerE/FixJ/NarL family proteins. Functional analysis of an aclR1 deletion 
mutant (ΔaclR1) in the free-living state showed that AclR1 positively regulated 
cell motility and flocculation but negatively regulated exopolysaccharide 
production, biofilm formation, and second messenger cyclic diguanylate 
(c-di-GMP)-related gene expression. In the symbiotic state, the ΔaclR1 mutant 
was defective in competitive colonization and nodulation on host plants. These 
results suggested that AclR1 could provide bacteria with the ability to compete 
effectively for symbiotic nodulation. Overall, our results show that the 
REC-LuxR-type regulator AclR1 regulates numerous phenotypes both in the 
free-living state and during host plant symbiosis.