Inactivation of SSM4, a New Saccharomyces Cerevisiae Gene, Suppresses MRNA Instability Due to Rna14 Mutations

Decay rates of mRNAs depend on many elements and among these, the role of the poly(A) tail is now well established. In the yeast Saccharomyces cerevisiae, thermosensitive mutations in two genes, RNA14 and RNA15, result in mRNAs having shorter poly(A) tails and reduced half-life. To identify other components interacting in the same process, we have used a genetic approach to isolate mutations that suppress the thermosensitivity of an rna14 mutant strain. Mutations in a single locus, named SSM4, not only suppress the cell growth phenotype but also the mRNA instability and extend the short mRNA poly(A) tails. The frequency of appearance and the recessive nature of these mutations suggested that the suppressor effect was probably due to a loss of function. We failed to clone the SSM4 gene directly by complementation, owing to its absence from gene banks; it later emerged that the gene is toxic to Escherichia coli, but we have nevertheless been able to clone the SSM4 sequence by Ty element transposition tagging. Disruption of the SSM4 gene does not affect cell viability and suppresses the rna14 mutant phenotypes. The protein encoded by the SSM4 gene has a calculated molecular mass of 151 kDa and does not contain any known motif or show homology with known proteins. The toxicity of the SSM4 gene in E. coli suggests that a direct biochemical activity is associated with the corresponding protein.