Comprehensive findings of functional modulations linked to structural intricacies in DNA binding for HapR/LuxR homologues from the Vibrionaceae family

LuxR/HapR homologues play important roles in the regulation of virulence and protease genes in pathogenic Vibrio species like V. cholerae, V. vulnificus, V. parahaemolyticus, etc. as well as providing physiological advantages such as resistance to protozoan grazing, motility and chitin-induced competence in the aqueous environments. They are commonly termed as the high cell density master regulators. They are highly conserved in sequence and structure among the Vibrios and can recognize and bind to common DNA promoters, like activation of protease production in V. cholerae by SmcR of V. vulnificus. Despite this conservation, critical differences occur due to amino acid divergence in their sequences. In the present study, we compare the degree of functional conservation among HapR/LuxR homologues selected from various clades of the Vibrionaceae family, using a hapR mutant V. cholerae strain as a model organism. We also delve into the structural intricacies of distant HapR/LuxR homologues, such as LuxR proteins of Photobacterium spp. and HapR of Vibrio mimicus, a close homologue of V. cholerae. Evolutionary implications arising from sequence divergence are also recorded in the structural changes through the crystal structure analysis. The study compared several structural constraints responsible for the variation in functions related to transcriptional regulation.