A TrmBL2-like transcription factor mediates the growth phase-dependent expression of halolysin SptA in a concentration-dependent manner in J7-2.

Unlabelled: To cope with a high-salinity environment, haloarchaea generally employ the twin-arginine translocation (Tat) pathway to transport secretory proteins across the cytoplasm membrane in a folded state, including Tat-dependent extracellular subtilases (halolysins) capable of autocatalytic activation. Some halolysins, such as SptA of J7-2, are produced at late-log phase to prevent premature enzyme activation and proteolytic damage of cellular proteins in haloarchaea; however, the regulation mechanism for growth phase-dependent expression of halolysins remains largely unknown. In this study, a DNA-protein pull-down assay was performed to identify the proteins binding to the 5'-flanking sequence of encoding halolysin SptA in strain J7-2, revealing a TrmBL2-like transcription factor (NgTrmBL2). The Δ mutant of strain J7-2 showed a sharp decrease in the production of SptA, suggesting that NgTrmBL2 positively regulates expression. The purified recombinant NgTrmBL2 mainly existed as a dimer although monomeric and higher-order oligomeric forms were detected by native-PAGE analysis. The results of electrophoretic mobility shift assays (EMSAs) showed that NgTrmBL2 binds to the 5'-flanking sequence of in a non-specific and concentration-dependent manner and exhibits an increased DNA-binding affinity with the increase in KCl concentration. Moreover, we found that a distal -regulatory element embedded in the neighboring upstream gene negatively regulates expression and thus participates in the growth phase-dependent biosynthesis of halolysin SptA.

Importance: Extracellular proteases play important roles in nutrient metabolism, processing of functional proteins, and antagonism of haloarchaea, but no transcription factor involved in regulating the expression of haloaechaeal extracellular protease has been reported yet. Here we report that a TrmBL2-like transcription factor (NgTrmBL2) mediates the growth phase-dependent expression of an extracellular protease, halolysin SptA, of haloarchaeon J7-2. In contrast to its hyperthermophilic archaeal homologs, which are generally considered to be global transcription repressors, NgTrmBL2 functions as a positive regulator for sptA expression. This study provides new clues about the transcriptional regulation mechanism of extracellular protease in haloarchaea and the functional diversity of archaeal TrmBL2.