Neural AADC mRNAs

Figure 11. Alternate promoters are used for production of the vertebrate neural and non-neural AADC mRNAs. Non-neural AADC transcription initiates at exon L1, whereas neural transcription initiates at exon N1. The non-neural mRNA splices from exon L1 to 2, since the 5 edge of exon N1 is a site of transcriptional initiation instead of a splice acceptor site. Translation initiates from the same AUG in exon 2 in both mRNAs, producing the same protein product in both tissue types. This scheme holds for both human and rat AADC, although the nomenclature of the exons differs. In rat AADC the exon N1 to 2 splice uses a splice acceptor site 5 bp downstream of the splice acceptor used for the exon L1 to 2 splice (Albert et al., 1992).

In Drosophila the two tissue-specific mRNAs are generated by alternative splicing of a single primary transcript (Fig. 9). In vertebrates the two tissue specific AADC transcripts are generated from two alternative promoters (Fig. 11) (Albert et al., 1992 Ichinose et al., 1992 Thai et al., 1993). In neural tissue transcription initiates from exon Nl, whereas in non-neural tissue transcription initiates from exon LI. This produces two distinct primary transcripts that are then spliced from the first exon (LI or Nl) to exon 2 to generate two tissue-specific mRNAs. Translation initiates within exon 2, such that the same AADC protein product is synthesized from both AADC mRNAs. 

Although there is a different pattern of exons spliced in the two AADC mRNAs, this apparent alternative splicing is a consequence of differences in transcription. In the non-neural mRNAthat initiates at exon LI,... 

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