Ustilago maydisTrf2 ensures genome stability by antagonizing Blm-mediated telomere recombination: fine-tuning DNA repair factor activity at telomeres through opposing regulations

Abstract

The duplex telomere repeat binding protein TRF2 stabilizes chromosome ends by suppressing DNA damage response and aberrant DNA repair. Herein we report our analysis of the Trf2 ortholog in Ustilago maydis (a basidiomycete fungus that bears strong resemblances to mammals with regard to telomere regulation), which underscores the suppression of Blm helicase as a key protective mechanism. Trf2 binds to Blm in vitro and inhibits Blm-mediated unwinding of telomeric DNA substrates. Notably, while the loss of Trf2 engenders growth arrest and a multiplicity of telomere aberrations, these defects are fully suppressed by concurrent deletion of blm and mre11 (but not other DNA repair factors). Over-expression of Trf2 induces telomere shortening through a pathway that is epistatic to blm delta. Moreover, over-expression of Blm alone triggers aberrant telomere recombination as evidenced by the accumulation ECTRs and C-circles, and these aberrations can be ameliorated by concurrent over-expression of Trf2. Together, these observations reveal a new mechanism for telomere protection by Trf2, namely through the repression of Blm activity. Interestingly, U. maydis harbors a second double-strand telomere repeat-binding protein (Tay1), which promotes Blm function at telomeres to ensure efficient replication. Our results thus point to opposing regulation of Blm helicase by telomere proteins as a strategy for fine-tuning Blm function such that this helicase is able to promote telomere maintenance without triggering telomere de-protection.