Abstract:[Background] Haloarchaea are able to survive in salt deposits for millions of years. Many haloarchaea produce extracellular subtilisin-like proteases (Halolysins). The Halolysins are folded in cells and transported across cytoplasmic or thylakoid membranes by Tat pathway. In addition, they mature automatically and most of them are produced at the late log phase and peak when the culture enters stationary phase. At the moment, the enzymatic properties, autocatalytic activation, and secretion mechanisms of Halolysins have been widely characterized. However, the physiological functions are rarely studied. Halolysin SptA is the major extracellular protease of Natrinema sp. J7-2. Previous studies showed that the growth phase-dependent production of SptA relies on the cooperative action of multiple cis-acting elements, allowing SptA to participate in the growth-phase transition of strain J7-2. SptA also contributes to the continued survival of strain J7-2 after the death phase. [Objective] To study the effect of SptA on long-term survival of strain J7-2. [Methods] strain J7-2 and ΔsptA1 mutant were cultured under nutrient-deficient, non-exogenous nutrient (liquid), and nutrient-rich (solid) conditions for a long time, respectively. The growth, survival, and SptA expression of the two strains were compared to further investigate the role of SptA. [Results] J7-2 strain produced more SptA under nutrient-deficient conditions, and number of viable J7-2 strain cells was significantly larger than that of viable ΔsptA1 cells after 33 days of growth under such conditions. strain J7-2 and ΔsptA1 underwent multiple events of cell division and cell death during long-term incubation in the absence of exogenous nutrients. The number of viable J7-2 strain cells was significantly larger than that of viable ΔsptA1 cells during the prolonged incubation (73-200 days). At the late stage of culture (160 days) on nutrient-rich solid plates, due to nutrient depletion, J7-2 strain benefited from SptA in terms of long-term survival by assimilating and utilizing the degradation products of proteins derived from dead cells.[Conclusion] The SptA-mediated cell death and degradation of dead cell-derived proteins enhance the long-term survival of J7-2 strain in response to nutrient starvation by helping J7-2 strain to scavenge dead cell-derived nutrients. This study provides new insight into the physiological role of Halolysins.
