A TRANSCRIPTIONAL REPRESSOR OBTAINED BY ALTERNATIVE TRANSLATION OF A TRINUCLEOTIDE REPEAT

Triplet nucleotide repeats are ubiquitous and rapidly evolving sequenc es in eucaryotic genomes. They are sporadically found in coding region s of transcription regulators where they become translated in differen t homopolymeric aminoacid (HPAA) stretches, depending on the local fra me. Poly(CAG) yields three different HPAAs (poly Gin, Ser or Ata). Cur rent sequence databases indicate a clear bias in the size and frequenc y of these HPAAs according to the rule: (Gln)n>(Ser)n>>(Ala)n. Aiming to understand the masons of this bias, we changed the translational re ading frame of the highly polymorphic CAG-repeat that normally encodes poly-Gin in the N-terminal portion of the rat glucocorticoid receptor (GR). The GR mutant in which the CAG repeat is translated to poly-Ala (called GR[Ala]) is incapable of transactivation, but maintains compe tence for hormone binding, nuclear translocation and specific DNA bind ing. We show that GR desactivation is obtained only when a very precis e threshold length of the repeat is reached. GR[Ala] displays a strong negative dominance when tested for transcriptional activation in vivo and may become useful for selective competition of receptor dependent activities in tissue culture cells and transgenic animals. We discuss the implications of our findings for the understanding of the evoluti onary behaviour of trinucleotide repeats in coding sequences.