Nucleoside transport is essential for maintaining intracellular nucleoside and nucleobase homeostasis for living cells. Here, we identified an uncharacterized GntR/HutC family transcriptional regulator, NagR2, renamed NupR (nucleoside permease regulator), that mainly controls nucleoside transport in the Bacillus thuringiensis BMB171 strain. The deletion or overexpression of nupR affected the bacteria's utilization of guanosine, adenosine, uridine, and cytidine rather than thymidine. We further demonstrated that zinc ion is an effector for the NupR, dissociating NupR from its target DNA. Moreover, the expression of nupR is inhibited by NupR, ComK, and PurR, while it is promoted by CcpA. Also, a purine riboswitch located in its 5' noncoding region influences the expression of nupR. Guanine is the ligand of the riboswitch, reducing the expression of nupR by terminating the transcription of nupR in advance. Hence, our results reveal an exquisite regulation mechanism enabling NupR to respond to multiple signals, control genes involved in nucleoside transport, and contribute to nucleoside substance utilization. Overall, this study provides essential clues for future studies exploring the function of the NupR homolog in other bacteria, such as Bacillus cereus, Bacillus anthracis, Klebsiella pneumoniae, and Streptococcus pneumoniae. IMPORTANCE The transport of nucleosides and their homeostasis within the cell are essential for growth and proliferation. Here, we have identified a novel transcription factor, NupR, which, to our knowledge, is the first GntR family transcription factor primarily involved in the regulation of nucleoside transport. Moreover, responding to diverse intracellular signals, NupR regulates nucleoside transport. It is vital for utilizing extracellular nucleosides and maintaining intracellular nucleoside homeostasis. NupR may also be involved in other pathways such as pH homeostasis, molybdenum cofactor biosynthesis, nitrate metabolism, and transport. In addition, nucleosides have various applications, such as antiviral drugs. Thus, the elucidation of the transport mechanism of nucleosides could be helpful for the construction of engineered strains for nucleoside production.
Keywords: GntR; NagR-like proteins; guanine; nucleoside transporter; nucleoside utilization; riboswitch.