RNA processing in Neurospora crassa mitochondria: transfer RNAs punctuate a large precursor transcript

The pattern of transcripts arising from a large contiguous portion of the Neurospora crassa mitochondrial genome and the processing of these transcripts have been investigated. Evidence is presented for the transcription of a single, at least 12.5 kb, precursor RNA that contains sequences corresponding to the apocytochrome b (cob), tRNACys, cytochrome oxidase subunit 1 (CO I), tRNAArg and unidentified reading frame (URF) 1 genes. The two tRNA sequences serve as punctuation signals for the cleavage of this large transcript. Processing at the tRNACys and tRNAArg sequences directly generates the proposed CO I mRNA, as well as the mature 3' end of the cob transcripts. Moreover, three other sites have been identified that are specifically involved in further 5' end processing events. One of the two 5' end processing sites of the cob transcript shares homology with the 5' end processing sites of the mitochondrial 19S and 25S rRNA precursors. PstI palindromes do not act as processing signals because six of these palindromic sequences are present in the cob and CO I mRNAs.