Structural basis of bifunctional CTP/dCTP synthase

Nucleotides are important molecules of life. According to the sugar, nucleotides can be divided into nucleotides and deoxynucleotides, which are the basic components of RNA and DNA respectively. CTP synthase (CTPS) catalyzes the last step in the de novo synthesis of CTP, which can form cytoophidia in cells from all three domains of life. We have recently found that CTPS combines with NTPs to form filaments, and solved the structures of CTPS bound with NTPs. Previous biochemical studies have shown that CTPS can also serve as dCTPS, i.e. CTPS/dCTPS can not only bind UTP, ATP and GTP to generate CTP, but also bind deoxynucleotides to generate dCTP. However, the structural basis of the bifunctional enzyme CTPS/dCTPS binding deoxynucleotide is not clear. In this study, we find that CTPS/dCTPS can form filaments bound with deoxynucleotides. Biochemically, we compare the binding and reaction characteristics of the corresponding nucleotides/deoxynucleotides and CTPS/dCTPS. Using cryo-electron microscopy, we solve the the structure of CTPS/dCTPS bound with deoxynucleotides at near-atomic resolution. This study not only provides a structural basis for understanding the catalysis and regulation of bifunctional CTPS/dCTPS, but also opens a door for further exploration the compartmentation of CTPS/dCTPS inside a cell. ### Competing Interest Statement The authors have declared no competing interest.