LARGE, COMPLEX MODULAR STRUCTURE OF A FISSION YEAST DNA-REPLICATION ORIGIN

Background: In the budding yeast, Saccharomyces cerevisiae, each DNA r eplication origin is associated with an autonomously replicating seque nce (ARS) element. Each element contains several modules, including an essential close match to the 11 base-pair (bp) ARS consensus sequence (ACS) and two or three short (< 20 bp) stimulatory motifs, within a s tretch of similar to 150 bp or less. To determine whether a similar or igin structure exists in the evolutionarily distant fission yeast, Sch izosaccharomyces pombe, we used deletion and linker substitution scann ing to identify the sequences important for the function of ars3002, a chromosomal replication origin. Results: We detected two large (30-55 bp) essential regions and several additional stimulatory sequences wi thin a 600 bp stretch of a restriction fragment containing ars3002. Th e two essential regions are similar to each other, and sequences simil ar to them are found in all known S. pombe ARS elements, suggesting th at one or both of them may represent the S. pombe equivalent of the S. cerevisiae ACS. Conclusions: Like S. cerevisiae origins, the S. pombe origin, ars3002, possesses a modular structure, but the number and si ze of modules is greater for ars3002, and ars3002 is larger than S. ce revisiae origins. These observations suggest that origin function in S . pombe requires more protein-DNA interactions than in S. cerevisiae.