NapR Regulates the Expression of Phosphoserine Aminotransferase SerC to Modulate Biofilm Formation and Resistance to Serine Stress of Mycobacteria

<i>Mycobacterium tuberculosis</i> is a formidable pathogen capable of establishing persistent infections within macrophages. To survive and thrive within the host environment, it has evolved intricate regulatory networks, including a diverse array of transcription factors that enable adaptation to various stresses encountered within the host. However, the mechanisms by which transcription factors regulate biofilm formation in <i>M. tuberculosis</i> remain incompletely understood. This study aimed to investigate the role of <i>serC</i>, encoding phosphoserine aminotransferase, and its regulation by NapR, a transcription factor, in mycobacterial physiology. NapR regulates <i>serC</i> through directly binding to its promoter. Notably, the regulatory effect and corresponding phenotypes vary due to distinct binding affinities of NapR for the <i>serC</i> promoter in different mycobacterial species. In <i>Mycobacterium smegmatis</i>, NapR_Msm positively regulates biofilm formation, growth on solid media, and the transition from microcolonies to microcolonies by activating <i>serC_Msm</i>. In the <i>BCG vaccine</i>, on the contrary, NapR_BCG represses <i>serC_BCG</i>, thus negatively regulating colony size and alleviating the growth inhibition caused by high concentrations of serine. Furthermore, proteomic analysis suggested NapR serves as a global transcriptional regulator in <i>BCG vaccine</i> strains by simultaneously modulating four metabolic pathways. These findings underscore the complex and strain-specific regulatory mechanisms governing serine metabolism in mycobacteria and provide valuable insights into the interplay between metabolism, gene regulation, and bacterial physiology.