CONSERVED ALTERNATIVE SPLICING IN THE 5'-UNTRANSLATED REGION OF THE MUSCLE-SPECIFIC ENOLASE GENE - PRIMARY STRUCTURE OF MESSENGER-RNAS, EXPRESSION AND INFLUENCE OF SECONDARY STRUCTURE ON THE TRANSLATION EFFICIENCY

We report here the isolation and characterization of cDNAs covering th e 5'-end region of mouse and rat mRNAs that encode the beta or muscle- specific isoform of the glycolytic enzyme enolase. As previously deter mined for humans, two classes of beta-enolase transcripts with distinc t sequences in their 5'-untranslated regions are present in both mouse and rat muscles. A mechanism of alternative splicing, conserved from mouse to man, generates the two forms of mRNA. Secondary-structure pre dictions indicated that, in all cases, a more stable secondary structu re could exist in the 5' end of the message with the longer leader. In vitro transcripts containing defined human or mouse 5'-untranslated s equences were obtained by fusion of the different cDNA clones and test ed for their relative translational efficiencies in rabbit reticulocyt e lysates. Transcripts containing the human long and short leader sequ ences showed differences in the translational rate, suggesting a role for the 5'-untranslated region in the regulation of translation. No de tectable difference was found between transcripts with the two distinc t mouse leader sequences. In addition, both transcripts are bound to p olysomes and are equally distributed along differently sized polysomes in C2C12 myogenic cells. The relative expression of the two spliced f orms in developing and adult muscle tissues by means of reverse transc ription and polymerase chain reaction did not show a stage-specific or a tissue-type-specific pattern. A putative functional role for the 5' -untranslated sequences of beta-enolase transcripts is discussed.