GENETIC-ANALYSIS OF AN ARS ELEMENT FROM THE FISSION YEAST SCHIZOSACCHAROMYCES-POMBE

ARS (autonomously replicating sequence) elements are DNA fragments tha t can function as origins of DNA replication in yeast, We report the f irst fine-structure analysis of ars1, an ARS element of the fission ye ast Schizosaccharomyces pombe. Characterization of a series of nested deletion mutations indicated that the minimal fragment of DNA encompas sing ars1 is surprisingly large, No fragment <650 bp retained signific ant ARS activity, Analysis of deletion and substitution mutations scan ning the entire minimal ars1 identified a single essential 50 bp fragm ent (segment 1). Only one other 50 bp mutation reduced activity as muc h as 5-fold and most deletions were without effect, Thus, the minimal ars1 is composed of two general types of genetic elements, a small seg ment that is absolutely required for efficient ARS activity and a much larger region that is tolerant of internal structural alterations. Hi gher resolution analysis of segment 1 defined a critical 30 bp A/T-ric h segment which appears to contain redundant genetic elements, Schizos accharomyces pombe ars1 promoted high frequency transformation in the budding yeast S.cerevisiae but this heterologous activity was not depe ndent on segment 1, Our analysis indicates that the functional element s required for ARS function in S.pombe and S.cerevisiae are clearly di fferent.