Abstract
Major porins are among the most abundant proteins embedded in the outer membrane (OM) of Gram-negative bacteria, playing crucial roles in maintenance of membrane structural integrity and OM permeability. Although many OM proteins (especially c-type cytochromes) in Shewanella oneidensis, a research model for respiratory versatility, have been extensively studied, physiological significance of major porins remains largely unexplored. In this study, we show that OmpS38 and OmpA are two major porins, neither of which is responsive to changes in osmolarity or contributes to the intrinsic resistance to β-lactam antibiotics. However, OmpS38 but not OmpA is largely involved in respiration of non-oxygen electron acceptors. We then provide evidence that expression of ompS38 is transcribed from two promoters, the major of which is favored under anaerobic conditions while the other appears constitutive. The major promoter is under the direct control of Crp, the master regulator dictating respiration. As a result, the increase in the level of OmpS38 correlates with an elevated activity in Crp under anaerobic conditions. In addition, we show that the activity of the major promoter is also affected by Fur, presumably indirectly, the transcription factor for iron-dependent gene expression.
Highlights
The outer membrane (OM) of Gram-negative bacteria is a highly asymmetric barrier that hinders the permeability of both hydrophilic and hydrophobic compounds[1]
OM fractions were prepared from mid-log phase cells, proteins in these fractions were resolved by SDS-PAGE, and visible bands were excised for protein determination by MS/ MS analysis
The discrepancy between migration and residue-based molecular weight is common to OmpA-like porins because of posttranslational modification (PTM)[52,53]
Summary
The outer membrane (OM) of Gram-negative bacteria is a highly asymmetric barrier that hinders the permeability of both hydrophilic and hydrophobic compounds[1]. To facilitate uptake of nutrients and other molecules for growth and diverse functions of the cell, the OM is embedded with a large number of channels formed by β -barrel proteins, including porins, substrate-specific transporters, and active transporters[2]. Shewanella, a group of facultative anaerobic γ -proteobacteria widely distributed in nature have received intensive attentions owing to their remarkably diverse respiratory capacities and the potential for environmental remediation and microbial fuel cell, which largely rely on extracellular electron transfer[18,19,20]. Insights into major porins are needed because of their abundance in the OM and role in transport of hydrophilic molecules, such as electron shuttles and some antibiotics. Further investigation reveals that OmpS38 is generally important in respiration of non-oxygen EAs and that the enhanced expression during anaerobic respiration largely relies on Crp activation
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