Defective splicing of mitochondrial rRNA in cytochrome-deficient nuclear mutants of Neurospora crassa.

Recent studies have shown that the gene encoding the large (25 S) mitochondrial rRNA of Neurospora crassa contains an intervening sequence of 2-2.5 kilobases that is not present in the mature 25S mitochondrial rRNA. Earlier studies had provided evidence that mitochondrial rRNAs in Neurospora are synthesized via a 32S precursor RNA that contains sequences for both the mature 19S and 25S RNA species. The present work shows that the intervening sequence is not present in 32S RNA. However, we have identified two temperature-sensitive nuclear mutants that fail to excise the intervening sequence at the nonpermissive temperature (37 degrees C). When grown at 37 degrees C, the mutants show decreased ratios of 25S to 19S RNA and accumulate a novel 35S RNA that appears to consist of 25S RNA plus most or all of the intervening sequence. The mutants are allelic but can be distinguished in temperature shift-up experiments, mitochondrial rRNA processing turning off more rapidly in one than in the other. These mutants should provide powerful new tools for studying RNA processing in eukaryotic cells.