Alternative modes of mRNA processing in a 3' splice site mutant of Neurospora crassa

The am8 mutant of Neurospora crassa is shown to have a double base-pair change, GTG for the normal TAG, at the 3' end of the second intron of the am (NADP-specific glutamate dehydrogenase, GDH) gene. The greater part of the mutant am transcript accumulates as two fragments hybridising to probes for sequences respectively upstream and downstream of the 5' end of the intron. Two processed transcripts approximating to normal full length mRNA were identified. In one the second intron was intact; in the other the second intron was spliced out through the use of an AAG sequence, 20 base-pairs into the third exon, as a 3' acceptor site. A GAG sequence, only four base-pairs downstream from the normal acceptor site, does not appear to be used.