Artificial Neural Network Modeling on PM10, PM2.5, and NO2 Concentrations between Two Megacities without a Lockdown in Korea, for the COVID-19 Pandemic Period of 2020.

The mutual relationship among daily averaged PM10, PM2.5, and NO2 concentrations in two megacities (Seoul and Busan) connected by the busiest highway in Korea was investigated using an artificial neural network model (ANN)-sigmoid function, for a novel coronavirus (COVID-19) pandemic period from 1 January to 31 December 2020. Daily and weekly mean concentrations of NO2 in 2020 under neither locked down cities, nor limitation of the activities of vehicles and people by the Korean Government have decreased by about 15%, and 12% in Seoul, and Busan cities, than the ones in 2019, respectively. PM 10 (PM2.5) concentration has also decreased by 15% (10%), and 12% (10%) in Seoul, and Busan, with a similar decline of NO2, causing an improvement in air quality in each city. Multilayer perception (MLP), which has a back-propagation training algorithm for a feed-forward artificial neural network technique with a sigmoid activation function was adopted to predict daily averaged PM10, PM2.5, and NO2 concentrations in two cities with their interplay. Root mean square error (RMSE) with the coefficient of determination (R2) evaluates the performance of the model between the predicted and measured values of daily mean PM10, PM2.5, and NO2, in Seoul were 2.251 with 0.882 (1.909 with 0.896; 1.913 with 0.892), 0.717 with 0.925 (0.955 with 0.930; 0.955 with 0.922), and 3.502 with 0.729 (2.808 with 0.746; 3.481 with 0.734), in 2 (5; 7) nodes in a single hidden layer. Similarly, they in Busan were 2.155 with 0.853 (1.519 with 0.896; 1.649 with 0.869), 0.692 with 0.914 (0.891 with 0.910; 1.211 with 0.883), and 2.747 with 0.667 (2.277 with 0.669; 2.137 with 0.689), respectively. The closeness of the predicted values to the observed ones shows a very high Pearson r correlation coefficient of over 0.932, except for 0.818 of NO2 in Busan. Modeling performance using IBM SPSS-v27 software on daily averaged PM10, PM2.5, and NO2 concentrations in each city were compared by scatter plots and their daily distributions between predicted and observed values.

A Dual Regulatory Role of the PhoU Protein in Salmonella Typhimurium.

Bacteria utilize two-component regulatory systems to sense and respond to their surroundings. Unlike other two-component systems that directly sense through a sensory domain in the histidine kinase (HK), the PhoB/PhoR two-component system requires additional proteins, including the PstSCAB phosphate transporter and the PhoU protein, to sense phosphate levels. Although PhoU is involved in phosphate signaling by connecting the PstSCAB transporter and PhoR histidine kinase, the mechanism by which PhoU controls expression of pho regulon genes has not yet been clearly understood. Here, we identified PhoU residues required for interacting with PhoR histidine kinase from the intracellular pathogen Salmonella enterica serovar Typhimurium. The PhoU Ala147 residue interacts with the PhoR PAS domain and is involved in repressing pho expression in high phosphate. Unexpectedly, the PhoU Arg184 residue interacts with the PhoR histidine kinase domain and is required for activating pho expression in low Mg2+ by increasing PhoR autophosphorylation, revealing its new function. The substitution of the Arg184 to Gly codon decreased Salmonella virulence both in macrophages and in mice, suggesting that PhoU's role in promoting PhoR autophosphorylation is required during Salmonella infection. IMPORTANCE Bacteria constantly sense and respond to their surroundings through two-component systems. In Gram-negative bacteria, phosphate sensing is mediated by the PhoB/PhoR two-component system with additional components, the PstSCAB phosphate transporter and the PhoU protein. PhoU, a regulatory protein that connects the PstSCAB phosphate transporter to the PhoB/PhoR two-component system, is believed to function as a negative regulator in phosphate signaling because the phoU deletion mutant loses the ability to repress pho expression in high phosphate. Using single amino acid substitutions in the intracellular pathogen Salmonella enterica serovar Typhimurium, PhoU turns out to control PhoR histidine kinase differently, depending on the conditions. The PhoU-PhoR PAS domain interaction is involved in repressing pho expression in high phosphate, whereas the PhoU-PhoR HK domain interaction is involved in activating autophosphorylation of PhoR histidine kinase in low Mg2+ and thus promotes Salmonella virulence. Therefore, PhoU appears to modulate phosphate signaling exquisitely according to external conditions.

The Salmonella virulence protein MgtC promotes phosphate uptake inside macrophages.

The MgtC virulence protein from the intracellular pathogen Salmonella enterica is required for its intramacrophage survival and virulence in mice and this requirement of MgtC is conserved in several intracellular pathogens including Mycobacterium tuberculosis. Despite its critical role in survival within macrophages, only a few molecular targets of the MgtC protein have been identified. Here, we report that MgtC targets PhoR histidine kinase and activates phosphate transport independently of the available phosphate concentration. A single amino acid substitution in PhoR prevents its binding to MgtC, thus abrogating MgtC-mediated phosphate transport. Surprisingly, the removal of MgtC's effect on the ability to transport phosphate renders Salmonella hypervirulent and decreases a non-replicating population inside macrophages, indicating that MgtC-mediated phosphate transport is required for normal Salmonella pathogenesis. This provides an example of a virulence protein directly activating a pathogen's phosphate transport inside host.

Elongation factor P restricts Salmonella's growth by controlling translation of a Mg2+ transporter gene during infection.

When a ribosome translates mRNA sequences, the ribosome often stalls at certain codons because it is hard to translate. Consecutive proline codons are such examples that induce ribosome stalling and elongation factor P (EF-P) is required for the stalled ribosome to continue translation at those consecutive proline codons. We found that EF-P is required for translation of the mgtB gene encoding a Mg2+ transporter in the mgtCBR virulence operon from the intracellular pathogen Salmonella enterica serovar Typhimurium. Salmonella lacking EF-P decreases MgtB protein levels in a manner dependent on consecutive proline codons located in the mgtB coding region despite increasing transcription of the mgtCBR operon via the mgtP open reading frame in the leader RNA, resulting in an altered ratio between MgtC and MgtB proteins within the operon. Substitution of the consecutive proline codons to alanine codons eliminates EF-P-mediated control of the mgtB gene during infection and thus contributes to Salmonella's survival inside macrophages where Salmonella experiences low levels of EF-P. This suggests that this pathogen utilizes a strategy to coordinate expression of virulence genes by an evolutionarily conserved translation factor.