PHYSICAL MAPPING OF ORIGINS OF REPLICATION IN THE FISSION YEAST SCHIZOSACCHAROMYCES-POMBE

We isolated four fragments from the Schizosaccharomyces pombe genome t hat mediate autonomous replication. A two-dimensional gel analysis rev ealed that in each case initiation could be mapped to within the S. po mbe sequences. In three of the fragments, initiation could be mapped t o one discrete location. In the fourth fragment, subcloning and two-di mensional gel analysis suggested that two discrete origins of replicat ion were located within 3 kb of each other. When in proximity, usually only one of these origins fired, suggesting origin interference. Two- dimensional gel analysis of the four origin fragments at their genomic locations demonstrated that each is used in the chromosomes, but in o nly a subset of cells or cell divisions. The S. pombe genome appears t o contain many discrete origins, not all of which fire in any given ce ll and some of which are closely spaced. Not I/Sfi I mapping of the fi ve origins from this and a previous study indicates that they are rand omly distributed throughout the genome and appear to be representative of chromosomal origins of replication in this organism. We compare th e features of S. pombe replication origins with those of S. cerevisiae and animal cells.