Abstract
ZEP1, a transverse filament (TF) protein, is the rice (Oryza sativa) homolog of Arabidopsis thaliana ZYP1. In the Tos17-insertional zep1 mutants, homologous chromosomes align along the entire length of the chromosome, but the synaptonemal complex is not assembled in early prophase I. Crossovers are well formed, and 12 bivalents could be detected from diakinesis to metaphase I, which leads to equal chromosomal segregation in anaphase I. Moreover, the number of crossovers has a tendency to be increased compared with that in the wild type. These phenomena are different from the TF mutants identified so far in other organisms. Chiasma terminalization of the bivalent, which occurs frequently in the wild type, seldom occurred in zep1. Transmission electron micrographs and immunodetection using an antibody against ZEP1 showed that ZEP1 is the central element of the synaptonemal complex. Although PAIR2 and MER3 were loaded normally in zep1, their dissociation was delayed severely compared with the wild type. In addition, ZEP1 is reloaded onto chromosomes in early microspores as the chromosome decondense, suggesting that ZEP1 might have other biological functions during this process.
Highlights
For all sexually propagating eukaryotes, meiosis is the crucial process of producing haploid gametes and consists of two rounds of chromosome division following a single round of DNA replication
The corresponding protein sequence of ZEP1 is most similar to ZYP1b (362/874 residues identical and 555/874 residues positive)
By means of RT-PCR, we revealed that ZEP1 is expressed mainly in young panicles and roots, and to a lesser extent in leaves and stems
Summary
For all sexually propagating eukaryotes, meiosis is the crucial process of producing haploid gametes and consists of two rounds of chromosome division following a single round of DNA replication. The assembly of SCs in budding yeast is closely coordinated with the initiation and maturation of homologous recombination events. ZMM complexes, which are required to implement interference-sensitive (class I) crossovers (COs), contain seven collaborating members, including ZIP1, ZIP2, ZIP3, ZIP4, MSH4, MSH5, and MER3. These proteins colocalize and are always present at the sites where SC polymerization initiates, so the ZMM proteins are referred to as the synapsis initiation complex and are markers of class I COs (Fung et al, 2004; Tsubouchi et al, 2006; Lynn et al, 2007). In mutants of non-ZIP1 ZMM components, ZIP1 always localizes to chromosomes as dots at the early stage of prophase I; while at pachytene, it forms polycomplexes that are never associated with chromosomes rather than the string-like signals
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