Expression of the open reading frames of a senescence-inducing, linear mitochondrial plasmid of Neurospora crassa

Abstract

Senescence-prone strains of Neurospora crassa from Aarey, India, harbor a linear mitochondrial plasmid, maranhar, which potentially encodes an RNA polymerase and a DNA polymerase (Court and Bertrand, 1992). To investigate the expression of the open reading frames (ORFs) of this plasmid, the mitochondria of the prototype of the senescence-prone strains, Aarey-1e, were analyzed for the presence of maranhar-specific transcripts and proteins. In addition to several low-abundance and small RNAs, two major large transcripts, each encompassing one of the long ORFs, were identified. The 5'-termini of these two transcripts are identical and coincide with a nucleotide sequence that is located in the terminal inverted repeats of the plasmid and partially resembles the promoter sequences of Neurospora mitochondrial genes. The 3' end of one of the plasmid transcripts also has been mapped. It is located within the DNA spacer between the two convergent ORFs and does not coincide with a conventional transcription-termination or RNA-processing signal. Two novel proteins, which may be the plasmid-encoded DNA and RNA polymerases, are synthesized in the mitochondria of the maranhar-containing strain. Although the termini of the plasmid are linked to terminal protein, a third plasmid-specific polypeptide was not observed. Transcripts encompassing three small ORFs in the maranhar nucleotide sequence also were identified. These RNAs occur in rather low copy numbers, and translation products that might correspond to the small ORFs were not detected in the mitochondria of the plasmid-containing strain.