MHF1 plays Fanconi anaemia complementation group M protein (FANCM)-dependent and FANCM-independent roles in DNA repair and homologous recombination in plants

Fanconi anaemia complementation group M protein (FANCM), a component of the human Fanconi anemia pathway, acts as DNA translocase that is essential during the repair of DNA interstrand cross-links. The DNA-damage-binding function of FANCM is strongly enhanced by the histone fold-containing FANCM-associated protein MHF1. We identified a single homologue of MHF1 in the genome of Arabidopsis thaliana. Similar to the loss of AtFANCM, the loss of AtMHF1 leads to several meiotic defects, such as chromosome bridges between bivalents and an unequal distribution of chromosomes. Moreover, MHF1, together with FANCM, is involved in interstrand cross-link repair in plants. This phenotype is detectable only in double mutants of the RecQ helicase and BLM homologue RECQ4A, which appears to function in a parallel pathway to the FANCM/MHF1 complex. However, in somatic cells, FANCM has an MHF1-independent function in replicative repair in a parallel pathway to the endonuclease MUS81. Furthermore, MHF1 is required for efficient somatic homologous recombination (HR) - a role antagonistic to FANCM. FANCM and RECQ4A define two parallel pathways of HR suppression in Arabidopsis. Hyperrecombination in the fancm but not the recq4A mutant can be abolished by MHF1 mutations. This finding indicates that MHF1 and FANCM act at different steps of a single, common, HR pathway.