We are working on a chromosomal imprinting mechanism responsible for the asymmetric division in the yeast Schizosaccharomyces pombe. The efficiency of this process leads to a clonal haploid cell population containing the same proportion of both mating-types.

Our previous work has shown that this process is initiated by a single strand DNA lesion at the mating-type locus, and involves DNA replication. The lesion is introduced during lagging strand synthesis, on only one of the two sister chromatids, is maintained during the entire length of the cell cycle and triggers mating-type switching during the following S-phase. The mechanism responsible for the formation and maintenance of this single strand DNA lesion is unknown. Recently, we have shown by the Meselson and Stahl (1958) approach that the mating-type locus is not replicated by the semi-conservative mode, but instead both DNA strands are synthesized de novo. This finding establishes a link between the phenotype and genotype in the process of asymmetric mating-type switching.

We are also studying the function of the sap1 gene, initially implicated in mating-type switching. Recently, we have shown that this gene plays an important role in the chromosomal cohesion/condensation processes which is essential for post-replicative recombination and segregation of the genetic material.

We wish to further analyze the mechanism responsible for single-strand break formation, strand exchanges during mating-type switching and resolution of the gene conversion intermediates. This work will use genetic, biochemical, molecular and cellular approaches, in collaboration with other research groups.

Another aspect of our work concerns the implication of DNA replication in the process of mating-type switching. Although much progress has been made towards understanding the replication initiation events, detailed molecular mechanism controlling elongation and termination remains unknown. A pilot approach using Chromatin Immuno-Precipitation (ChIP) coupled with DNA combing methodology will be developed in collaboration with the A. Bensimon group (Pasteur Institute).