Activation of c-src neuron-specific splicing by an unusual RNA element in vivo and in vitro.

A conserved positive-acting RNA sequence was found to be required for the neuron-specific splicing of the mouse c-src N1 exon. The sequence lies in the intron between exons N1 and 4, close to the N1 donor site. Normally, only the neural-specific splicing of exon N1 required this sequence. When the intron downstream of N1 was shortened, splicing at the constitutive exon 4 acceptor also became dependent on the activating sequence. The neuronal and nonneuronal patterns of src splicing were reconstituted in vitro. HeLa cell extracts spliced exon 4 to exon 3, skipping exon N1. Weri-1 retinoblastoma cell extracts spliced exon 4 to exon N1 as well as to exon 3. Both patterns of splicing were dependent on the activating sequence. A 123 nt RNA containing just the activating sequence specifically inhibited both patterns of src splicing, indicating that factors bound to the activator were required for its effects.