Two distinct temperature-sensitive alleles at the elav locus of Drosophila are suppressed nonsense mutations of the same tryptophan codon.

The Drosophila gene elav encodes a 483-amino-acid-long nuclear RNA binding protein required for normal neuronal differentiation and maintenance. We molecularly analyzed the three known viable alleles of the gene, namely elavts1, elavFliJ1, and elavFliJ2, which manifest temperature-sensitive phenotypes. The modification of the elavFliJ1 allele corresponds to the change of glycine426 (GGA) into a glutamic acid (GAA). Surprisingly, elavts1 and elavFliJ2 were both found to have tryptophan419 (TGG) changed into two different stop codons, TAG and TGA, respectively. Unexpectedly, protein analysis from elavts1 and elavFliJ2 reveals not only the predicted 45-kD truncated ELAV protein due to translational truncation, but also a predominant full-size 50-kD ELAV protein, both at permissive and nonpermissive temperatures. The full-length protein present in elavts1 and elavFliJ2 can a priori be explained by one of several mechanisms leading to functional suppression of the nonsense mutation or by detection of a previously unrecognized ELAV isoform of similar size resulting from alternative splicing and unaffected by the stop codon. Experiments described in this article support the functional suppression of the nonsense mutation as the mechanism responsible for the full-length protein.