Contribution of a specific XRE family transcriptional regulator to the oleaginous phenotype in rhodococci

Abstract

Oleagenicity is a property attributed to some microorganisms capable of accumulating high levels of intracellular lipids within the so-called lipid droplets (LDs). Some species of the Rhodococcus genus, such as R. opacus and R. jostii, are able to accumulate triacylglycerols (TAG) up to 60% or more of their cellular dry weight. For this reason, oleaginous rhodococci are promising microbial cell factories for the production of lipids to be used as fuels and oleochemicals. Although several genes involved in TAG biosynthesis and accumulation have been well described, it is not clear yet how these processes are regulated. Global and specific transcriptional regulators (TRs) contribute to the oleaginous phenotype in Rhodococcus. Among specific TRs, a XRE family transcriptional regulator (TR) is associated with the lipid droplet ontogeny through regulation of a structural protein coding gene. In this work, we study the role of this specific TR on lipid metabolism in oleaginous rhodococci at the physiological and molecular level. Bioinformatic analysis revealed the occurrence of this regulator only in actinobacteria. In addition, the occurrence of putative TR boxes into the promoters regions varied between oleaginous Rhodococcus strains and non-oleaginous strains. Docking studies revealed putative interactions of this specific TR with palmitic acid. In vitro and in vivo assays confirmed that the TR binding capacity to DNA is controlled by long chain fatty acids or their acyl-CoA derivatives. Glutaraldehyde (GT) cross-linker assay and limited proteolysis analysis revealed that long chain fatty acids induce oligomerization and conformational changes of TR, respectively. Furthermore, putative binding sites for this TR within upstream regions of genes coding for a lipase, an acyl-CoA dehydrogenase and the fatty acid synthase complex (FASI) were found and validated by EMSA and RT-PCR assays. Finally, deregulation of the TR levels by overexpression of the corresponding gene was used as a strategy to improved TAG biosynthesis and lipid recovery for biotechnological purposes under rich nitrogen conditions. We propose a model in which the activity of this TR is controlled by fatty acyl-CoA pools in cells according to the nutritional conditions of the environment. In addition, this protein participates in the regulatory network controlling lipid metabolism and lipid droplet formation in oleaginous rhodococci.