Molecular Organization of Yeast Chromosome I - Control of Meiotic Pairing and Recombination

Reciprocal recombination (crossing over) between homologous chromosomes (homologues) is believed to be required for proper meiosis I segregation. The mechanisms that control recombination so that each pair of homologues undergoes crossing over are not known. In Saccharomyces cerevisiae and humans, small chromosomes have higher meiotic reciprocal recombination rates (cM/kb) than large chromosomes, and in S. cerevisiae, rates of reciprocal recombination have been shown to respond directly to chromosome size. One set of recent experiments, however, suggested that size-dependent control of crossing over was not a general feature in all strains or on all chromosomes. As it has been proposed that size-dependent control of recombination is an essential feature of the mechanism guaranteeing crossing over between homologues, the investigator plans to find out if it indeed occurs in all strains and chromosomes and, if it does not, to begin to determine why it might not be occurring. The decreased rates of recombination on large chromosomes have been proposed to be due to increased crossover interference. Interference is defined by the apparent inhibition of additional meiotic reciprocal recombination events observed near the site of a crossover. The molecular mechanism of interference and how it might respond to chromosome size are not known. It has been suggested that the density of meiotic recombination-inducing double-strand break (DSB) sites is greater on small chromosomes than on large ones. If this is indeed the case, then chromosome size-dependent control of recombination (and, perhaps, interference) might act by controlling DSB formation. To test this idea, the investigator plans to analyze DSB formation on chromosome I constructs of different sizes. The investigator has also discovered that the most distal euchromatic DNA undergoes meiotic recombination at a higher rate than more interior sequences, suggesting that chromosome position also regulates recombination. The investigator will determine whether chromosome position does actually affect recombination rates.

The next part of this project involves the meiotic synaptonemal complex (SC). The precise function of this structure is not known, but it has been proposed that it may serve as a regulator of meiotic recombination. The investigator has produced a functional, fluorescent ZIP1-GFP fusion protein that labels SCs in living cells. He has used this protein to show that SCs undergo dynamic movements and changes in their distribution. The role of these movements is unknown but they could play a role in facilitating recombination. He now proposes to label axial elements with REC8-CFP and, through the use of fluorescence video microscopy, examine the dynamics of pairing and SC movements. These studies should provide new insights into the molecular mechanisms controlling recombination and chromosome synapsis, processes that lead to proper meiotic chromosome segregation.

In most species, proper chromosome segregation is essential for the production of normal gametes (e.g. sperm, eggs, pollen, and ascospores). Identifying mechanisms that ensure proper chromosome segregation will provide a better understanding of how meiotic divisions occur. A better understanding of these mechanisms could lead to greatly improved plant breeding and animal husbandry. Having this research carried out at UMDNJ in Newark will also be important for the scientific education of both graduate and medical students at that institution, and for the undergraduates from neighboring institutions (NJIT, Rutgers) who will actively participate in the project. Furthermore, UMDNJ's location has been instrumental in the investigator's significant success in enabling gifted underrepresented minority students to participate in research. He will continue this effort with the aim of producing outstanding scientists from UMDNJ's environs. The opening of Newark's Science High School one block from his laboratory should further increase his and his laboratory's impact upon Newark's secondary school population as well.