THE MEF2A 3'-UNTRANSLATED REGION FUNCTIONS AS A CIS-ACTING TRANSLATIONAL REPRESSOR

Myocyte enhancer factor 2 (MEF2) proteins serve as important muscle tr anscription factors, In addition, MEF2 proteins have been shown to pot entiate the activity of other cell-type specific transcription factors found in muscle and brain tissue, While transcripts for MEF2 factors are widely expressed in a variety of cells and tissues, MEF2 proteins and binding activity are largely restricted to skeletal, smooth, and c ardiac muscle and to brain. This disparity between MEF2 protein and mR NA expression suggests that translational control may play an importan t role in regulating MEF2 expression, In an effort to identify sequenc es within the MEF2A message which control translation, we isolated the mouse MEF2A 3' untranslated region (UTR) and fused it to the chloramp henicol acetyltransferase (CAT) reporter gene. Here, we show by CAT as say that the MEF2A 3' UTR dramatically inhibits CAT gene expression in vivo and that this inhibition is due to an internal region within the highly conserved 3' UTR, RNase protection analyses demonstrated that the steady-state level of CAT mRNA produced in vivo was not affected b y fusion of the MEF2A 3' UTR, indicating that the inhibition of CAT ac tivity resulted from translational repression, Furthermore, fusion of the MEF2A 3' UTR to CAT inhibited translation in vitro in rabbit retic ulocyte lysates. We also show that the translational repression mediat ed by the 3' UTR of MEF2A is regulated during muscle cell differentiat ion, As muscle cells in culture differentiate, the translational inhib ition caused by the MEF2A 3' UTR is relaxed, These results demonstrate that the MEF2A 3' UTR functions as a cis-acting translational repress or both in vitro and in vivo and suggest that this repression may cont ribute to the tissue-restricted expression and binding activity of MEF 2A.