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In budding yeast, Rif1 negatively regulates telomere length, but the mechanism of this regulation has remained elusive. Previous work identified several functional domains of Rif1, but none of these has been shown to mediate telomere length. To define Rif1 domains responsible for telomere regulation, we localized truncations of Rif1 to a single specific telomere and measured telomere length of that telomere compared to bulk telomeres. We found that a domain in the N-terminus containing HEAT repeats, Rif1177-996, was sufficient for length regulation when tethered to the telomere. Charged residues in this region were previously proposed to mediate DNA binding. We found that mutation of these residues disrupted telomere length regulation even when Rif1 was tethered to the telomere. Mutation of other conserved residues in this region, which were not predicted to interact with DNA, also disrupted telomere length maintenance, while mutation of conserved residues distal to this region did not. Our data suggest that conserved amino acids in the region from 436 to 577 play a functional role in telomere length regulation, which is separate from their proposed DNA binding function. We propose that the Rif1 HEAT repeats region represents a protein-protein binding interface that mediates telomere length regulation.

To examine the established link between DNA replication and telomere length, we tested whether firing of telomeric origins would cause telomere lengthening. We found that RIF1 mutants that block Protein Phosphatase 1 (PP1) binding activated telomeric origins but did not elongate telomeres. In a second approach, we found overexpression of ∆N-Dbf4 and Cdc7 increased DDK activity and activated telomeric origins, yet telomere length was unchanged. We tested a third mechanism to activate origins using the sld3-A mcm5-bob1 mutant that de-regulates the pre-replication complex, and again saw no change in telomere length. Finally, we tested whether mutations in RIF1 that cause telomere elongation would affect origin firing. We found that neither rif1-∆1322 nor rif1HOOK affected firing of telomeric origins. We conclude that telomeric origin firing does not cause telomere elongation, and the role of Rif1 in regulating origin firing is separable from its role in regulating telomere length.

In this work, the arc resistance of electrical contacts of the compositions WC–Cu, Cr3C2–Cu, and WC–Cr3C2–Cu (with a mass ratio of WC : Cr3C2 = 1 : 1) was studied in comparison with the industrial contact D30V70, and an analysis was made of the dependence of functional properties on the composition and contact structures. Experimental samples of electrical contacts were obtained by liquid-phase impregnation of noncompacted carbide powders with copper at low-frequency (~80 Hz) vibration of the crucible with alloy components for 10 min in a resistance furnace in an atmosphere of flowing argon. Prepared samples were tested using equipment that simulated the operation of an AC contactor (170 A, 50 V, 1000–10 000 arcing cycles). Their functional properties (hardness, arc wear, contact resistance) were studied. The evolution of the structure was studied using optical and electron microscopy. It is shown that alloys containing WC or WC + Cr3C2 have stable values of contact resistance (up to 10 000 arcing cycles). Alloy WC57Cu43 (wt %) has the highest arc resistance, which not only is comparable with the industrial alloy but also surpasses it in long-term tests, despite the lower content of the arc-resistant phase. In contacts of the Cr3C2–Cu composition, the contact resistance increases and grows most sharply after 4000 arcing cycles. In contacts made of bicarbide alloy WC–Cr3C2–Cu, the degree of increase in contact resistance is similar to the industrial alloy, but the average values are higher. The wear layers of monocarbide alloys are highly oxidized. In a bicarbide alloy, there is no oxygen in the composition of such a layer. The composition of this alloy is represented by solid solutions of metals and their carbides, and the structure is finely dispersed. The mechanisms of arc resistance in electrical contacts of various compositions are discussed. [ABSTRACT FROM AUTHOR]