Absence of yKu/Hdf1 but not myosin-like proteins alters chromosome dynamics during prophase I in yeast.

Meiosis is central to the formation of haploid gametes or spores in that it segregates homologous chromosomes and halves the chromosome number. A prerequisite of this genome bisection is the pairing of homologous chromosomes during the first meiotic prophase. When budding yeast cells are induced to undergo meiosis, this has profound consequences for nuclear structure: after premeiotic DNA replication centromeres disperse, while telomeres move about the nuclear periphery and temporarily cluster during the leptotene/zygotene transition (bouquet stage) of the prophase to first meiotic division. In vegetative cells, Hdf1p (yKu) and the myosin-like proteins Mlp1p and Mlp2p have been suggested to contribute to the organization of silent chromatin, tethering of telomeres to the nuclear periphery, DNA repair, and telomere maintenance. Here, we investigated by molecular cytology whether yKu and Mlp proteins contribute to telomere and chromosome dynamics in meiosis. It was found that mlp1 Delta mlp2 Delta double-mutant cells undergo centromere dispersion, telomere clustering, homologue pairing, and sporulation like wild type. On the other hand, cells deficient for yKu underwent meiosis-specific chromosomal events with a delay, while they eventually sporulated like wild type. These results suggest that the absence of yKu not only affects vegetative nuclear architecture (Laroche et al., 1998) but also interferes with the ordered occurrence of chromosome dynamics during first meiotic prophase.