“…The gene expression and promoter–reporter analyses revealed that HSAF production is controlled primarily at the gene expression level (Chen et al ., 2017). Six transcription regulators that affect the expression of the HSAF biosynthesis operon have been identified so far, including five positive regulators, PilR, Clp, LysR, LarR and CdgL, and one negative regulator, LetR (Wang et al ., 2014; Chen et al ., 2017; Su et al ., 2017; Wang et al ., 2017; Su et al ., 2018; Xu et al ., 2018; Han et al ., 2020; Xu et al ., 2021b). Clp, LysR, LarR and LetR are transcription factors that directly bind to the promoter of the HSAF biosynthesis operson (Fig.…”