No abstract

The chromatin structures of the telomeric and subtelomeric regions on chromosomal DNA molecules in Saccharomyces cerevisiae were analyzed using micrococcal nuclease and DNase I. The subtelomeric repeats X and Y' were assembled in nucleosomes. However, the terminal tracts of Cj.jA repeats were protein protected in a particle larger than a nucleosome herein called a telosome. The proximal boundary of the telosome was a DNase I hypersensitive site. This boundary between the telosome and adjacent nucleosomes was completely accessible to Escherichia coli dam methylase when this enzyme was expressed in yeast, whereas a site 250 bp internal to the telomeric repeats was relatively inaccessible. Telosomes could be cleaved from chromosome ends with nuclease and solubilized as protein-DNA complexes. Immunoprecipitation of chromosomal telosomes with antiserum to the RAPl protein indicated that RAPl was one component of isolated telosomes. Thus, the termini of chromosomal DNA molecules in yeast are assembled in a non-nucleosomal structure encompassing the entire terminal C1.3A tract. This structure is separated from adjacent nucleosomes by a region of DNA that is highly accessible to enzymes.

The mitogen-activated protein kinase (MAPK) cascade is required for mitogenesis in somatic mammalian cells and is activated by a wide variety of oncogenic stimuli. Specific roles for this signaling module in growth were dissected by inhibiting MAPK kinase 1 (MAPKK1) activity in highly synchronized NIH 3T3 cells. In addition to the known role of this kinase in cell-cycle entry from G 0 , the level of MAPKK activity was observed to affect the kinetics of progression through both the G 1 and G 2 phases of the cell cycle in NIH 3T3 cells. Ectopic expression of dominant-negative forms of MAPKK1, which was previously shown to inhibit G 0 /G 1 progression, was found to also delay progression of cells through G 2 . In addition, treatment of cells with the specific MAPKK inhibitor PD 98059 during a synchronous S phase arrested the cells in the following G 2 phase. These data demonstrate a novel role for the MAPK cascade in progression from G 2 into mitosis in NIH 3T3 cells.