SARS ARE CIS DNA ELEMENTS OF CHROMOSOME DYNAMICS - SYNTHESIS OF A SARREPRESSOR PROTEIN

SARs are candidate DNA elements for defining the bases of chromatin lo ops and possibly for serving as cis elements of chromosome dynamics. S ARs contain numerous A tracts, whose altered DNA structure is recogniz ed by cooperatively interacting proteins such as topoisomerase II. We constructed multi-AT hook (MATH) proteins and demonstrate that they sp ecifically bind the clustered A tracts of SARs in chromatin and chromo somes. They are also potent inhibitors of chromosome assembly in mitot ic Xenopus extracts, demonstrating the importance of SARs in this proc ess. Titration of SARs with MATH20 (20 hooks) blocks shape determinati on of chromatids but not chromatin condensation per se. SARs are also required for shape maintenance of chromosomes. If MATH20 is added afte r formation of chromatids, they collapse and are reshaped by an active , mitotic process into spherical chromatid balls.