Telomere Length Regulation and Telomeric Chromatin Require the Nonsense-Mediated mRNA Decay Pathway

ABSTRACT

Rap1p localization factor 4 (RLF4) is aSaccharomyces cerevisiae gene that was identified in a screen for mutants that affect telomere function and alter the localization of the telomere binding protein Rap1p. In rlf4mutants, telomeric silencing is reduced and telomere DNA tracts are shorter, indicating that RLF4 is required for both the establishment and/or maintenance of telomeric chromatin and for the control of telomere length. In this paper, we demonstrate thatRLF4 is allelic to NMD2/UPF2, a gene required for the nonsense-mediated mRNA decay (NMD) pathway (Y. Cui, K. W. Hagan, S. Zhang, and S. W. Peltz, Mol. Cell. Biol. 9:423–436, 1995, and F. He and A. Jacobson, Genes Dev. 9:437–454, 1995). The NMD pathway, which requires Nmd2p/Rlf4p together with two other proteins, (Upf1p and Upf3p), targets nonsense messages for degradation in the cytoplasm by the exoribonuclease Xrn1p. Deletion of UPF1and UPF3 caused telomere-associated defects like those caused by rlf4 mutations, implying that the NMD pathway, rather than an NMD-independent function of Nmd2p/Rlf4p, is required for telomere functions. In addition, telomere length regulation required Xrn1p but not Rat1p, a nuclear exoribonuclease with functional similarity to Xrn1p (A. W. Johnson, Mol. Cell. Biol. 17:6122–6130, 1997). In contrast, telomere-associated defects were not observed in pan2, pan3, or pan2 pan3 strains, which are defective in the intrinsic deadenylation-dependent decay of normal (as opposed to nonsense) mRNAs. Thus, loss of the NMD pathway specifically causes defects at telomeres, demonstrating a physiological requirement for the NMD pathway in normal cell functions. We propose a model in which the NMD pathway regulates the levels of specific mRNAs that are important for telomere functions.

ACKNOWLEDGMENTS

We thank Maryam Gerami-Nejad for technical assistance and David Gartner and Mark Sanders of the University of Minnesota Imaging Center for assistance with digital imaging. We thank Feng He, Allen Jacobson, Alan Sachs, and Michael Culbertson for providing strains and plasmids. We thank Steve Johnston, Cathy Asleson, Michael Lelivelt, and Jeff Dahlseid for critical reading of the manuscript and many helpful suggestions.

This work was supported by a grant from the National Institutes of Health (GM38636) to J.B.

ADDENDUM IN PROOF

Recently, CTF13, a gene important for centromere function, was found to be regulated, indirectly, by the NMD pathway (J. N. Dahlseid, J. Puziss, R. L. Shirley, A. L. Atkin, P. Hieter, and M. R. Culbertson, Genetics, in press).