SILENT CHROMATIN IN YEAST - AN ORCHESTRATED MEDLEY FEATURING SIR3P

Extensive regions of chromosomes can be transcriptionally repressed th rough silencing mechanisms mediated by complex chromatin structures. O ne of the most refined molecular portraits of silenced chromatin comes from studies of the silent mating-type loci and telomeres of S. cerev isiae. In this budding yeast, the Sir3p silent information regulator e merges as a critically important silencing component that interacts wi th nucleosomes and other silencing proteins. Not only is it essential for silencing, but Sir3p is also capable of spreading silenced chromat in when its dosage is increased. Sir3p is a target of mitogen-activate d protein (MAP) kinase cascade regulation and has significant similari ty to the Orc1 p subunit of the DNA replication origin recognition com plex. Thus, in concert with other silencing proteins, Sir3p appears po ised to respond to cellular signals and reprogram silencing through re plication-associated assembly of repressive chromatin structures. (C) 1998 John Wiley & Sons, Inc.