Mechanisms of survival against desiccation and associated stresses of the soil-residing actinomycete Rhodococcus jostii strain RHA1

Actinomycetes are an abundant bacterial group in soil, with a critical role in the decomposition of organic matter. Rhodococcus jostii strain RHA1 is of particular interest to the field of bioremediation because it can degrade a broad range of organic compounds, both natural and xenobiotic. Understanding the factors contributing to the desiccation resistance of RHA1 will enrich our basic knowledge of this common soil stress and may help advance bioremediation technologies for contaminated soils subject to droughts. Here I report the first transcriptomic analysis of a Gram-positive bacterium during desiccation. Filtered RHA1 cells incubated at either low relative humidity, as an air-drying treatment, or high relative humidity, as a control, were transcriptionally profiled over a comprehensive time series. Also, the morphology of RHA1 cells was characterized by cryofixation scanning electron microscopy during each treatment. Desiccation resulted in a transcriptional response of 819 differentially regulated genes, 8-times more than in the control. Included among the highly up-regulated desiccation-specific genes was dps1 (induced 33-fold), encoding an oxidative stress protection protein which has not previously been directly associated with desiccation, as well as sigF3 (induced 58-fold), encoding a sigma factor possibly involved in the regulatory response to desiccation. RHA1 mutants with dps1 or both of its dps homologs deleted were challenged with oxidative stressors under a variety of assay conditions. The mutants were also exposed to physiological stresses that generate reactive oxygen species intracellularly, including desiccation. In all cases, the dps− mutants did not have impaired oxidative stress resistance – a novel finding with respect to bacterial dps-null strains. Additionally, the RHA1 dps-null mutant did not have substantially lower survival compared to the wild type when challenged with metal toxicity or DNA-damaging agents or when they were cocultured through multiple cycles of starvation. Nevertheless, expression of RHA1 dps1 in an Escherichia coli dps– mutant restored its hydrogen peroxide resistance. Purified RHA1 Dps1 was shown to have ferroxidase activity and thereby to protect DNA from oxidative damage. The general insensitivity of the RHA1 dps-null mutant may be representative of a large group of Actinobacteria for which robust oxidative stress tolerance is an important adaptation.Science, Faculty ofMicrobiology and Immunology, Department ofGraduat