The mRNA poly(A) tail serves different purposes, including
the facilitation of nuclear export, mRNA stabilization, efficient
translation, and, finally, specific degradation. The
posttranscriptional addition of a poly(A) tail depends on sequence
motifs in the 3′ untranslated region (3′ UTR) of
the mRNA and a complex trans-acting protein machinery.
In this study, we have replaced the 3′ UTR of the yeast
TRP4 gene with sequences encoding a hammerhead ribozyme
that efficiently cleaves itself in vivo. Expression of the
TRP4-ribozyme allele resulted in the accumulation of
a nonpolyadenylated mRNA. Cells expressing the
TRP4-ribozyme mRNA showed a reduced growth rate due
to a reduction in Trp4p enzyme activity. The reduction in enzyme
activity was not caused by inefficient mRNA export from the
nucleus or mRNA destabilization. Rather, analyses of mRNA
association with polyribosomes indicate that translation of
the ribozyme-containing mRNA is impaired. This translational
defect allows sufficient synthesis of Trp4p to support growth
of trp4 cells, but is, nevertheless, of such magnitude
as to activate the general control network of amino acid biosynthesis.