The General Stress Response Factor EcfG Regulates Expression of the C-2 Hopanoid Methylase HpnP in Rhodopseudomonas palustris TIE-1

Abstract

c Lipid molecules preserved in sedimentary rocks facilitate the reconstruction of events that have shaped the evolution of the Earth's biosphere. A key limitation for the interpretation of many of these molecular fossils is that their biological roles are still poorly understood. Here, we use Rhodopseudomonas palustris TIE-1 to identify factors that induce biosynthesis of 2-methyl hopanoids (2-MeBHPs), progenitors of 2-methyl hopanes, one of the most abundant biomarkers in the rock record. This is the first dissection of the regulation of hpnP, the gene encoding the C-2 hopanoid methylase, at the molecular level. We demonstrate that EcfG, the general stress response factor of alphaproteobacteria, regulates expression of hpnP under a variety of challenges, including high temperature, pH stress, and presence of nonionic osmolytes. Although higher hpnP transcription levels did not always result in higher amounts of total methylated hopanoids, the fraction of a particular kind of hopanoid, 2-methyl bacteriohopanetetrol, was consistently higher in the presence of most stressors in the wild type, but not in the ⌬ecfG mutant, supporting a beneficial role for 2-MeBHPs in stress tolerance. The ⌬hpnP mutant, however, did not exhibit a growth defect under the stress conditions tested except in acidic medium. This indicates that the inability to make 2-MeBHPs under most of these conditions can readily be compensated. Although stress is necessary to regulate 2-MeBHP production, the specific conditions under which 2-MeBHP biosynthesis is essential remain to be determined.

Hopanoid lipids: from membranes to plant–bacteria interactions

“…Under these conditions, hopanoid content is typically increased, and it has long been proposed that hopanoids are regulated by canonical stress response pathways 88 . The alphaprotobacterial general stress response pathway regulates the hpnP gene in R. palustris TIE-1, although shc appears to be unregulated in this organism 89 . Regulation of hopanoids in other bacteria has not been studied.…”

Diverse capacity for 2-methylhopanoid production correlates with a specific ecological niche

“…This factor induces hpnP expression in response to osmotic stress in R. palustris (Kulkarni et al, 2013); osmolyte production by plants in the rhizosphere is well documented, and in some cases has been shown to promote plant-microbe symbioses (Miller and Wood, 1996;Khamar et al, 2010).…”

Bacteriohopanepolyols in tropical soils and sediments from the Congo River catchment area

“…Additionally, BHPs may be involved in a response to environmental stress (e.g. Kulkarni et al, 2013). The difference in BHP distributions between climate zones (Fig.…”