Analysis of the Low salt transcriptome in Haloferax volcanii

Archaea are well known for inhabiting areas of our planet that are usually hostile for any form of life, hence they are often termed as "extremophiles". Although also found in common soil or the digestive tract of vertebrates, some species thrive on acidic or alkaline grounds, in submarine black smokers with temperatures surpassing 100°C or live in aquatic areas with a very high salinity.

Haloferax volcanii is a halophilic and mesophilic archaeon that was first discovered in the 1930s and isolated from the hypersaline environment of the Dead Sea. Its optimal growth conditions are at 42 °C in around 2 M NaCl and complex nutrient medium, however it can also grow in different synthetic media and at a wide range of temperatures and NaCl concentrations. While NaCl concentrations up to 0.7 M inhibit growth in synthetic glucose media, a concentration of 0.9 M is sufficient to sustain growth at a reduced rate.

Here we show for the first time a transcriptome-wide analysis of differential gene expression of cells grown under low salt condition. We cultivated cells at an optimal NaCl concentration of 2.1 M in glucose media and compared the expression profile of these to cells grown in 0.9 M NaCl after 26 and 68 h.

Using RNA-Seq, we identified a multitude of differentially regulated genes and gene-clusters that seem to be of importance either as a fast "stress-response" (after 26 h) to the change of ion concentrations in the environment or to enable continuous growth under these conditions (68 h). We are currently generating in-frame deletion mutants of selected genes/gene clusters and will investigate the impact of these deletions on growth under 0.9 M NaCl. To this end, two of seven tested deletion mutants (HVO_0772, annotated as a NP_1176A family transcription regulator and HVO_1863, annotated as a conserved hypothetical protein) showed a severe growth defect under low salt while having only a minor under normal salt, indicating that these genes are involved in adaption to a low salt environment.