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All organisms are subject to stress from environmental changes, and understanding the specific molecular responses to allow cells to maintain viability is a fundamental biological problem. We explore this process in the halophilic microbe Haloferax volcanii. We hypothesize that they have evolved so that a subset of their population consists of cells prepared to survive dramatic stress events. As a first step to examine how these 'persisting' cells endure, we used RNA sequencing to examine transcriptome differences in cells that survived osmotic shock compared to unstressed cells. Our data revealed that approximately half of the protein-coding genes exhibited statistically significant differences in transcriptome abundance with a nearly equal distribution of more abundant and less abundant transcripts. There were about twice the number of transcripts exhibiting a 10-fold or greater decrease (5.3% of protein-coding genes) in abundance than those with the same magnitude of increased abundance (2.7% of protein-coding genes). These results may indicate that cells primed for survival reduce production of proteins required for active metabolism. We are currently conducting experiments to determine if deletion or overexpression of some of these genes affects susceptibility of H. volcanii to osmotic shock. Our work may provide insight into why some cells in a microbial population survive stress events despite being genetically identical to cells that are killed. [ABSTRACT FROM AUTHOR]

Halophilic Archaea are a distinctive pink color due to a carotenoid pigment called bacterioruberin. To sense or utilize light, many halophilic Archaea also produce rhodopsins, complexes of opsin proteins with a retinal prosthetic group. Both bacterioruberin and retinal are synthesized from isoprenoid precursors, with lycopene as the last shared intermediate. We previously described a regulatory mechanism by which Halobacterium salinarum bacterioopsin and Haloarcula vallismortis cruxopsin inhibit bacterioruberin synthesis catalyzed by lycopene elongase. In this work, we found that opsins in all three major Halobacteria clades inhibit bacterioruberin synthesis, suggesting that this regulatory mechanism existed in the common Halobacteria ancestor. Halophilic Archaea , which are generally heterotrophic and aerobic, likely evolved from an autotrophic, anaerobic methanogenic ancestor by acquiring many genes from Bacteria via lateral gene transfer. These bacterial "imports" include genes encoding opsins and lycopene elongases. To determine if opsins from Bacteria inhibit bacterioruberin synthesis, we tested bacterial opsins and found that an opsin from Curtobacterium , in the Actinobacteria phylum, inhibits bacterioruberin synthesis catalyzed by its own lycopene elongase, as well as that catalyzed by several archaeal enzymes. We also determined that the lycopene elongase from Halococcus salifodinae , a species from a family of Halobacteria lacking opsin homologs, retained the capacity to be inhibited by opsins. Together, our results indicate that opsin-mediated inhibition of bacterioruberin biosynthesis is a widely distributed mechanism found in both Archaea and Bacteria , possibly predating the divergence of the two domains. Further analysis may provide insight into the acquisition and evolution of the genes and their host species. IMPORTANCE All organisms use a variety of mechanisms to allocate limited resources to match their needs in their current environment. Here, we explore how halophilic microbes use a novel mechanism to allow efficient production of rhodopsin, a complex of an opsin protein and a retinal prosthetic group. We previously demonstrated that Halobacterium salinarum bacterioopsin directs available resources toward retinal by inhibiting synthesis of bacterioruberin, a molecule that shares precursors with retinal. In this work, we show that this mechanism can be carried out by proteins from halophilic Archaea that are not closely related to H. salinarum and those in at least one species of Bacteria Therefore, opsin-mediated inhibition of bacterioruberin synthesis may be a highly conserved, ancient regulatory mechanism., (Copyright © 2018 American Society for Microbiology.)