Transcriptionally silent chromatin localizes to the nuclear periphery, which provides a special microenvironment for gene repression. A variety of nuclear membrane proteins interact with repressed chromatin, yet the functional role of these interactions remains poorly understood. Here, we show that, in Schizosaccharomyces pombe, the nuclear membrane protein Lem2 associates with chromatin and mediates silencing and heterochromatin localization. Unexpectedly, we found that these functions can be separated and assigned to different structural domains within Lem2, excluding a simple tethering mechanism. Chromatin association and tethering of centromeres to the periphery are mediated by the N-terminal LEM (LAP2-Emerin-MAN1) domain of Lem2, whereas telomere anchoring and heterochromatin silencing require exclusively its conserved C-terminal MSC (MAN1-Src1 C-terminal) domain. Particularly, silencing by Lem2 is epistatic with the Snf2/HDAC (histone deacetylase) repressor complex SHREC at telomeres, while its necessity can be bypassed by deleting Epe1, a JmjC protein with anti-silencing activity. Furthermore, we found that loss of Lem2 reduces heterochromatin association of SHREC, which is accompanied by increased binding of Epe1. This reveals a critical function of Lem2 in coordinating these antagonistic factors at heterochromatin. The distinct silencing and localization functions mediated by Lem2 suggest that these conserved LEM-containing proteins go beyond simple tethering to play active roles in perinuclear silencing.

In nature, Saccharomyces yeasts manifest a number of adaptive responses to overcome adverse environments such as filamentation, invasive growth, flocculation and adherence to solid surfaces. Certain Saccharomyces wild yeasts, namely ''flor yeasts,'' have also acquired the ability to form a buoyant biofilm at the broth surface. Here we report that mutations in a single gene, identified as FLO11, separate these ''floating'' yeasts from their nonfloating relatives. We have determined that the capability to form a self-supporting biofilm at the liquid surface is largely dependent on two changes in the FLO11 gene. First, we identified a 111-nt deletion within a repression region of the FLO11 promoter that significantly increases FLO11 gene expression. Secondly, we found rearrangements within the central tandem repeat domain of the coding region that yield a more hydrophobic Flo11p variant. Together, these mutations result in dramatic increase in cell surface hydrophobicity, which in turn confers these yeasts the ability to float by surface tension, an adaptive mechanism to gain direct access to oxygen within oxygen-poor liquid environments.adaptive mechanism ͉ buoyant biofilm ͉ yeast hydrophobicity

Adhesins play a central role in the cellular response of eukaryotic microorganisms to their host environment. In pathogens such as Candida spp. and other fungi, adhesins are responsible for adherence to mammalian tissues, and in Saccharomyces spp. yeasts also confer adherence to solid surfaces and to other yeast cells. The analysis of FLO11, the main adhesin identified in Saccharomyces cerevisiae, has revealed complex mechanisms, involving both genetic and epigenetic regulation, governing the expression of this critical gene. We designed a genomewide screen to identify new regulators of this pivotal adhesin in budding yeasts. We took advantage of a specific FLO11 allele that confers very high levels of FLO11 expression to wild ''flor'' strains of S. cerevisiae. We screened for mutants that abrogated the increased FLO11 expression of this allele using the loss of the characteristic fluffy-colony phenotype and a reporter plasmid containing GFP controlled by the same FLO11 promoter. Using this approach, we isolated several genes whose function was essential to maintain the expression of FLO11. In addition to previously characterized activators, we identified a number of novel FLO11 activators, which reveal the pH response pathway and chromatin-remodeling complexes as central elements involved in FLO11 activation.