SCEP3 initiates synapsis and implements crossover interference in Arabidopsis

<p dir="ltr">The synaptonemal complex (SC) is a meiosis-specific tripartite proteinaceous structure that regulates the number and positions of crossovers (COs). Here we characterize SCEP3, a new Arabidopsis SC component that is essential for CO assurance, promoting positive CO interference and preventing negative CO interference. SCEP3 localizes to the chromosome axes as numerous foci at leptotene, of which a small proportion cluster as large foci that initiate synapsis. SCEP3 then relocates to the central region of the SC as ZYP1 polymerizes. In the absence of SCEP3, homologues align but do not synapse. In the scep3 mutants, COs increase in number towards the chromosome ends and are more likely to cluster together. SCEP3 encodes an 801-amino-acid intrinsically disordered protein that is structurally similar to SIX6OS1 in mammals and SYP-4 in nematodes, containing phenylalanine repeats at the amino terminus and a carboxy-terminal coiled-coil, suggesting that it is a fundamentally conserved SC component across kingdoms.</p>