Academic journal article Genetics

Hybrid Mitochondrial Plasmids from Senescence Suppressor Isolates of Neurospora Intermedia

Academic journal article Genetics

Hybrid Mitochondrial Plasmids from Senescence Suppressor Isolates of Neurospora Intermedia

Article excerpt

ABSTRACT

We analyzed several natural suppressor isolates of the pKalilo-based fungal senescence syndrome of Neurospora intermedia. The pKalilo plasmid did not increase in titer in these isolates. Nor did it show integration "de novo." In at least two of the senescence suppressor isolates, pKalilo had formed stable recombinants with other mitochondrial elements. pKalilo/mtDNA recombination junctions were complete and appeared to have been formed via a nonhomologous recombination mechanism. Further analysis revealed that pKalilo had recombined a novel, 2.6-kb cryptic mitochondrial retroplasmid, similar to the mitochondrial retroplasmid pTHR1 from Trichoderma harzianum and retroplasmids of the "Varkud" homology group. The recombinant molecules consisted of pKalilo, the novel element, and short intervening stretches of mtDNA. The latter stretches clearly corresponded to "in vivo" mitochondrial cDNA, suggesting that the molecules had formed via the action of a template-switching reverse transcriptase. We discuss how different types of mitochondrial plasmids interact and how their detrimental effect on the host may be suppressed.

AGING or senescence in fungi has been a longstanding matter of debate because fungi, being typical modular organisms and thus lacking a clear distinction between germ line and soma, are not expected to age and die. Even though most fungi indeed appear to be extremely long lived, exceptions, however, exist. Cases of senescence or senescence-like phenomena have, for example, been reported within the ascomycetous genera Neurospora and Podospora (GRIFFITHS 1992).

Senescence in Neurospora can almost invariably be ascribed to the presence of a single mitochondrial plasmid, such as one of the linear invertron-type mitochondrial plasmids pKalilo (BERTRAND et al. 1985, 1986; MYERS et al. 1989) and pMaranhar (COURT et al. 1991) or one of the circular mitochondrial retroplasmids pVarkud and pMauriceville (AKINS et al. 1989). These plasmids are all able to integrate into the mitochondrial genome of their hosts and interfere with mitochondrial function, which is allegedly the cause of death of the cultures that carry them. Often they are found close to or within the genes encoding the mitochondrial rRNAs (BERTRAND et al. 1985; BERTRAND and GRIFFITHS 1989; MYERS et al. 1989; CHIANG et al. 1994; M. F. P. M. Maas, unpublished results). The invertron-type plasmids allegedly integrate via short DNA sequence homology (SSH) with the mitochondrial target site, whereas the retroplasmids integrate via a reverse transcription step. pVarkud and pMauricevile both contain an open reading frame (ORF) encoding a reverse transcriptase (RT) that can generate hybrid mitochondrial cDNA via RNA template switching. Following hybrid cDNA formation, the latter plasmids may integrate via homologous recombination (CHIANG et al. 1994).

Little is known about suppressors of "plasmid-based" senescence in Neurospora. On the basis of the analogy with senescence in Podospora anserina, one might expect that modifications of the respiratory chain would act as suppressors of senescence (DUFOUR and LARSSON 2004). Thus far, however, there is no evidence to support this. In the case of pKalilo, there are reports of nuclear-encoded suppressors: In octads collected from crosses between senescing and nonsenescing laboratory strains of Neurospora, several cases were found in which senescence and "immortality" segregated in a four-to-four ratio (GRIFFITHS et al. 1992). In one case the titer of pKalilo was reduced to barely detectable levels, whereas in another case the titer of this plasmid was high but stable and associated with copies of the plasmid integrated into the mitochondrial chromosome. These nuclear-encoded suppressors of pKalilo-based senescence were not characterized any further. A search for suppressors among natural Neurospora isolates was purportedly unsuccessful (Debets et al. 1995). In the latter search, however, not every pKalilo-carrying isolate senesced, and suppressors may hence have been present. …

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