Mismatch Repair Protein Msh6^Tt Is Necessary for Nuclear Division and Gametogenesis in Tetrahymena thermophila

DNA mismatch repair (MMR) improves replication accuracy by up to three orders of magnitude. The MutS protein in E. coli or its eukaryotic homolog, the MutSα (Msh2-Msh6) complex, recognizes base mismatches and initiates the mismatch repair mechanism. Msh6 is an essential protein for assembling the heterodimeric complex. However, the function of the Msh6 subunit remains elusive. Tetrahymena undergoes multiple DNA replication and nuclear division processes, including mitosis, amitosis, and meiosis. Here, we found that Msh6^Tt localized in the macronucleus (MAC) and the micronucleus (MIC) during the vegetative growth stage and starvation. During the conjugation stage, Msh6^Tt only localized in MICs and newly developing MACs. MSH6 knockout led to aberrant nuclear division during vegetative growth. The MSH6 mutants were resistant to treatment with the DNA alkylating agent methyl methanesulfonate (MMS) compared to wild type cells. MSH6 knockout affected micronuclear meiosis and gametogenesis during the conjugation stage. Furthermore, Msh6^Tt interacted with Msh2^Tt and MMR-independent factors. Downregulation of MSH2 expression affected the stability of Msh6^Tt. In addition, MSH6 knockout led to the upregulated expression of several MSH6 homologs at different developmental stages. Msh6^Tt is involved in macronuclear amitosis, micronuclear mitosis, micronuclear meiosis, and gametogenesis in Tetrahymena.