Multiple Functions of Yeast Telomeric Heterochromatin

Sabrina Venditti, Glauco Di Stefano and Ernesto Di Mauro*

Fondazione Istituto Pasteur-Fondazione Cenci Bolognetti, c/o Dipartimento di Genetica e Biologia Molecolare, Università di Roma »La Sapienza«, P.le A. Moro 5, 00185 Roma, Italy

Article history:
Received October 18, 2000 
Accepted November 29, 2000

Key words:
telomeres, silencing, heterochromatin, Saccharomyces cerevisiae

Summary:
Heterochromatin-like structure at yeast telomeres is involved in the regulation of several important biological processes. Telomeres facilitate end replication, stabilize chromosomes and display many features of higher eukaryotes heterochromatin. In eukaryotes telomeres exert position effect variegation on genes located in their immediate vicinity, a phenomenon called transcriptional silencing. Many examples of silencing are known in yeast, among which the HM mating type loci. In all cases the establishement and maintenance of silencing is essentially achieved through the organization of chromatin of higher order structure, involving many protein factors. The Sir proteins and Rap1p are the main components of this compact and inaccessible structure, and are usually found concentrated at a defined number of loci at the nuclear periphery. A large body of experimental evidences has lead to the definition of a tridimensional model that takes into account all the interactions among the Sir proteins, Rap1p and other additional factors. In spite of the compactness of heterochromatin, though, the Sir protein complex seems to be able to relocate to other loci in several instances, suggesting the existence of subnuclear compartments where differential regulation events may take place. In addition to the regulation of silencing, the Sir proteins, Rap1p and Rap1p interacting factors are involved in different ways in the regulation of telomeric organization, controlling the number of (C_1-3A)_n repeats and the number and type of subtelomeric middle-repetitive elements present at each chromosome end. These mechanisms may involve either telomerase or recombination-mediated pathways. Few cases of naturally occurring telomeric silencing have been described, and the possible biological significance of this phenomenon still awaits clarification. Currently, the number of genes somehow connected with the »on« and »off« state of telomere- linked genes is increasing, shading light on the variety of cellular processes affected by the integrity and stability of telomeric heterochromatin.

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