Role and Regulation of σ s in General Resistance Conferred by Low-Shear Simulated Microgravity in Escherichia coli

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Abstract
Life on Earth evolved in the presence of gravity, and thus it is of interest from the perspective of space exploration to determine if diminished gravity affects biological processes. Cultivation of Escherichia coli under low-shear simulated microgravity (SMG) conditions resulted in enhanced stress resistance in both exponential- and stationary-phase cells, making the latter superresistant. Given that microgravity of space and SMG also compromise human immune response, this phenomenon constitutes a potential threat to astronauts. As low-shear environments are encountered by pathogens on Earth as well, SMG-conferred resistance is also relevant to controlling infectious disease on this planet. The SMG effect resembles the general stress response on Earth, which makes bacteria resistant to multiple stresses; this response is σ s dependent, irrespective of the growth phase. However, SMG-induced increased resistance was dependent on σ s only in stationary phase, being independent of this sigma factor in exponential phase. σ s concentration was some 30% lower in exponential-phase SMG cells than in normal gravity cells but was twofold higher in stationary-phase SMG cells. While SMG affected σ s synthesis at all levels of control, the main reasons for the differential effect of this gravity condition on σ s levels were that it rendered the sigma protein less stable in exponential phase and increased rpoS mRNA translational efficiency. Since σ s regulatory processes are influenced by mRNA and protein-folding patterns, the data suggest that SMG may affect these configurations.

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