Background/Objectives: The Structural Maintenance of Chromosomes (SMC) proteins form essential heterocomplexes for the preservation of DNA structure and its functions, and hence cell viability. The SMC5/6 dimer is assembled by direct interactions of ATP heads via the kleisin NSE4 bridge and by SMC hinges. The structure might be interrupted by a single point mutation within a conserved motif of the SMC6-hinge. We describe the phenomena associated with the impairment of the SMC5/6 complex with morphology, repair of DNA double strand breaks (DSB), mutagenesis, recombination and gene targeting (GT) in the moss Physcomitrium patens (P. patens). Methods: Using CRISPR/Cas9-directed oligonucleotide replacement, we have introduced two close G to R point mutations in the hinge domain of SMC6 of P. patens and show that both mutations are not toxic and allow viability of mutant lines. Results: The G514R mutation fully prevents the interaction of SMC6 not only with SMC5, but also with NSE5 and NSE6, while the mutation at G517R has no effect. The Ppsmc6_G514R line has aberrant morphology, spontaneous and bleomycin-induced mutagenesis, and maintenance of the number of rDNA copies. The most unique feature is the interference with gene targeting (GT), which is completely abolished. In contrast, the Ppsmc6_G517R line is close to WT in many aspects. Surprisingly, both mutations have no direct effect on the rate of DSB repair in dividing and differentiated cells. Conclusions: Abolished interactions of SMC6 with SMC5 and NSE5,6 partners, which allow DSB repair, but impair other repair and recombination functions, suggests also regulatory role for SMC6.

Institute of Experimental Botany Czech Academy of Sciences Rozvojová 263 165 00 Prague Czech Republic

Mendel Centre for Plant Genomics and Proteomics Central European Institute of Technology Masaryk University Kamenice 5 625 00 Brno Czech Republic

National Center for Biomolecular Research Faculty of Science Masaryk University Kamenice 5 625 00 Brno Czech Republic

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