Transcription of the kalilo linear senescence plasmid from Neurospora intermedia

The 8.6-kb kalilo linear mitochondrial plasmid of Neurospora intermedia was found to give rise to multiple transcripts of 8.6, 4.4, 4.0, 1.3, 1.2, and 0.9 kb. Plasmid transcripts were found in all the developmental stages tested and were present in cells with and without plasmid integration into mtDNA. The 4.4- and 4.0-kb transcripts corresponded to the two known ORFs in size, position, and polarity. An analysis of RNA fractions has confirmed the mitochondrial location of kalilo. The kalilo-specific RNA appears to be selectively unstable in affected strains of N. intermedia; this may be a general property of linear plasmid RNA. Transcripts are most likely initiated from near the termini of the linear plasmid. Primer extension and sequencing showed a clear 5' RNA end at nucleotide 101. However, the sequence in this region does not show homology with any known mitochondrial, plasmid, or nuclear promoter. The sequence does show homology with the tip of the plasmid and marks the border of a long series of direct repeats. Possibly the plasmid RNA polymerase may be bifunctional, recognizing sequences at the plasmid tip as well as the promoter.