Chapter 4. Evaluating the control of mRNA decay in fission yeast.

Abnormalities in rates of mRNA decay can lead to changes in steady-state levels of transcripts, which in turn can result in changes in protein production and abnormal phenotypes. For example, mice deficient in the gene encoding tristetraprolin (TTP), a tandem CCCH zinc finger domain protein, develop a complex syndrome that includes wasting, arthritis, and myeloid hyperplasia, all secondary to elevated levels of tumor necrosis factor (TNF). This in turn reflects elevated levels of TNF mRNA, which is a direct "target" of TTP binding and TTP-promoted deadenylation and decay. Three TTP-like proteins are expressed in human and four in mice, all of which bind mRNA and control transcript decay. In contrast, the Schizosaccharomyces pombe genome contains only one TTP-like protein, named Zfs1. Microarray analysis revealed that S. pombe cells deficient in zfs1 overexpress the arz1 mRNA, which has several ideal TTP-like binding sites in its 3'-untranslated region (UTR). We used the "no message in thiamine (nmt)" repressible system, in which thiamine rapidly shuts off gene transcription, to evaluate the relative stability of the arz1 mRNA in wild-type and zfs1-deficient cells. We found that the arz1 mRNA decayed much more rapidly in the presence of endogenous zfs1 than in its absence. The nmt system also proved useful for the study of mRNA sequence elements that are essential for interactions with zfs1, which eventually results in accelerated transcript decay. These studies illustrate the utility of the S. pombe nmt system for evaluating protein-mRNA interactions that affect mRNA decay in vivo and provide an alternative to the use of transcription inhibitors or heat-sensitive polymerase promoters that are used more commonly to evaluate mRNA decay in Saccharomyces cerevesiae. We hope to use this convenient experimental system to unravel the mechanism by which TTP family members, in this and other organisms, bind to mRNAs and promote their instability.