The telomere binding proteins in yeast and their role in telomere maintenance

Telomeres are the end-structures of linear chromosome which are crucial for maintaining genome integrity. The telomeres prevent the DNA-repair machinery from recognizing the chromosome ends as double-strand breaks and from end-to-end fusions. The telomeres are composed by short repetitive units of TG-rich DNA and associated specialized telomere-binding proteins.

Due to that replication only can occur unidirectional the telomeres become shorter as cells proliferate. This functions as a “biological clock” for the cells which will senescence after a certain number of divisions because of the shortening of the telomeres. In stem cells, germ cells and unicellular organism with linear chromosomes this shortening is counteracted by a ribonucleoprotein enzyme called telomerase. Telomerase adds on repetitive DNA units to the outermost end of the telomere according to a template sequence within an intrinsic RNA-subunit of the enzyme. Telomerase prefers to extend short telomeres over long telomeres hence the enzyme is able to scrutinize between telomeres of different lengths. This ability is partly controlled by the telomere associated proteins which can make the telomere end more or less accessible for telomerase depending on the length of the telomere.

All known telomeres end with a single-stranded 3’overhang which server as a substrate for the telomerase enzyme as well as a binding site for the Cdc13 protein. The duplex telomeric DNA is bound by the Rap1 protein, which is a negative regulator of telomere length. My doctoral thesis focuses on understanding how the telomere binding proteins bind to DNA and how they interact with each other. I have determined the terminal nucleotide of the complementary AC-rich 5’strand, which will dictate the composition of the telomeric double-stranded- and single-stranded junction (ds-ss junction). In addition, I have in detail analyzed how the telomere-binding proteins Rap1 and Cdc13 bind to telomeric DNA and how they assemble around the ds-ss junction. Moreover I have also characterized the telomerase enzyme and its ability to prime and elongate different types of substrates. These findings have led to a better understanding of the molecular details that involve the intriguing absolute ends of chromosomes.