Caenorhabditis elegans RMI2 functional homolog-2 (RMIF-2) and RMI1 (RMH-1) have both overlapping and distinct meiotic functions within the BTR complex.

Maria Velkova,Verena Jantsch,Maria Rosaria Dello Stritto,Nicola Silva,Markus Hartl,Alexander Schleiffer,Pierre Barraud

doi:10.1371/journal.pgen.1009663

Abstract

Homologous recombination is a high-fidelity repair pathway for DNA double-strand breaks employed during both mitotic and meiotic cell divisions. Such repair can lead to genetic exchange, originating from crossover (CO) generation. In mitosis, COs are suppressed to prevent sister chromatid exchange. Here, the BTR complex, consisting of the Bloom helicase (HIM-6 in worms), topoisomerase 3 (TOP-3), and the RMI1 (RMH-1 and RMH-2) and RMI2 scaffolding proteins, is essential for dismantling joint DNA molecules to form non-crossovers (NCOs) via decatenation. In contrast, in meiosis COs are essential for accurate chromosome segregation and the BTR complex plays distinct roles in CO and NCO generation at different steps in meiotic recombination. RMI2 stabilizes the RMI1 scaffolding protein, and lack of RMI2 in mitosis leads to elevated sister chromatid exchange, as observed upon RMI1 knockdown. However, much less is known about the involvement of RMI2 in meiotic recombination. So far, RMI2 homologs have been found in vertebrates and plants, but not in lower organisms such as Drosophila, yeast, or worms. We report the identification of the Caenorhabditis elegans functional homolog of RMI2, which we named RMIF-2. The protein shows a dynamic localization pattern to recombination foci during meiotic prophase I and concentration into recombination foci is mutually dependent on other BTR complex proteins. Comparative analysis of the rmif-2 and rmh-1 phenotypes revealed numerous commonalities, including in regulating CO formation and directing COs toward chromosome arms. Surprisingly, the prevalence of heterologous recombination was several fold lower in the rmif-2 mutant, suggesting that RMIF-2 may be dispensable or less strictly required for some BTR complex-mediated activities during meiosis.

Highlights

Damage-induced DNA double-strand breaks (DSBs) pose a threat to genome integrity
Bloom syndrome is caused by mutations in proteins of the BTR complex and the clinical characteristics are growth deficiency, short stature, skin photosensitivity, and increased cancer predisposition
Characteristic features are the presence of increased sister chromatid exchange on chromosomes; unresolved DNA recombination intermediates that eventually cause genome instability; and erroneous DNA repair by heterologous recombination, which can trigger translocations and chromosomal rearrangements

Summary

Introduction

Highfidelity repair via homologous recombination (HR) is employed during both mitotic and meiotic cell cycles. It involves the generation of 30 overhang ends by DNA resection and their stabilization by the single-stranded DNA-binding protein RPA (replication protein A) (RPA-1 in worms). After DNA synthesis and second-end capture, DNA joint molecules are generated. These can be processed to produce crossovers (COs), which result in the reciprocal exchange of large regions of chromosomes [1]. In meiosis, where Spo11-mediated DSBs are induced via a highly regulated program, crossing-over and cohesion establish a physical tether between homologous chromosomes, which greatly aids their correct segregation in meiotic anaphase I and drives genetic variability. To ensure at least one CO per chromosome pair, excess DSBs are introduced and those that do not form the CO are repaired to form NCOs [2]

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