Phenotypes Associated With Fungal Linear Plasmids

Linear plasmids were detected in a number of plant pathogenic filamentous fungi; however, they apparently do not contribute to a pathogen's detrimental properties but rather persist as neutral passengers (Griffiths 1995).

In Neurospora, insertion of the linear plasmids kalilo and maranhar into the host's mitochondrial DNA routinely results in onset of senescence due to mitochondrial malfunction, ultimately leading to death (reviewed by Griffiths 1992; Bertrand 2000). Kalilo and maranhar insertion causes large inverted repetitions of mitochondrial DNA adjacent to the insertion site (Bertrand et al. 1985, 1986; Court et al. 1991). As a specific (defective) mitochondrial

RNA pol

DNA pol

Neurospora crassa maranhar (7052 bp)

RNA pol

DNA pol

Pleurotus ostreatus pMLP1

DNA pol

Morchella conica pMC3-2 (6044 bp)

DNA pol

Ascobolus immersus pAI1

1 kb

Fig. 2 Schematic representation of the structure and coding capacity of mitochondrial linear plasmids of filamentous fungi (a selection). Arrows indicate ORFs and their transcriptional direction; TPs are depicted as filled circles; TIRs correspond to filled triangles

DNA variant with inserted plasmids (IS-kalDNA or IS-marDNA) dominates in a particular senescent culture, it has apparently originated from a single insertion event (Myers et al. 1989). Supposedly, functionally impaired IS-kalDNA or IS-marDNA carrying mitochondria trigger mitochondrial replication to compensate for the defect and thus gradually replace wild-type mitochondria dividing at normal rates (Myers et al. 1989; Griffiths 1995).

Integration of a linear plasmid (pAL2-1) into the mitochondrial DNA has also been reported for Podospora anserina (AL2); however, in marked contrast to Neurospora, insertion of pAL2-1 resulted in a prolonged life span of the regularly aging fungus (Hermanns et al. 1994, 1995). Since other isolates of P. anserina carrying pAL2-1 homologous elements senesce normally (van der Gaag et al. 1998), such a phenomenon was apparently strain specific. Moreover, when P. anserina served as a model for studying the lifetime prolonging effect of calorie restriction (Maas et al. 2004), it was disclosed that life span (under calorie restriction) is indeed shortened by pAL2-1; hence, as for kalilo and maranhar, pAL2-1 was considered to induce an early onset of senescence (Maas et al. 2004, 2005).

In summary and by way of conclusion, most fungal linear plasmids appear to be cryptic elements, and occasionally they are associated with a molecular disease leading to massive accumulation of defective mitochondria which is—by the way—accompanied by a vast propagation of the inserted plasmid copy. Thus, fungal linear plasmids represent selfish or parasitic DNAs with no discernible positive effects on their hosts.

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