Circular Retroplasmids

The Mauriceville plasmid of Neurospora crassa is a covalently closed circular DNA of approximately 3.6 kb. It has high sequence identity (97-99%) with the Varkud, Maddur-1, and Maddur-2 plasmids of a related species, N. intermedia (Nargang et al. 1984; Akins et al. 1988; D'Souza et al. 2005). The 2.6 kb pThrl plasmid of Trichoderma harzianum is the only other circular retroplasmid reported outside of Neurospora species (Antal et al. 2002; Fig. 1). Recently, a possible related retroplasmid was identified among PCR-amplified products of mitochondrial DNAs of Neurospora intermedia isolates (Maas et al. 2007). An autonomously replicating form of the putative hybrid plasmid was not reported; thus it remains to be seen whether this represents a new linear or circular plasmid form. The Mauriceville retroplasmid has a single large open reading frame (ORF) that encodes a 710 amino acid polypeptide that has reverse transcriptase activity (Kuiper and Lambowitz 1988; Kuiper et al. 1990; Table 1). The plasmid replication cycle has been extensively characterized and in vitro studies demonstrate that the Mauriceville reverse transcriptase (pMaur-RT) has unique enzymatic properties (Wang and Lambowitz 1993a). Isolates containing Mauriceville or Varkud are phenotypically similar to wildtype strains, but unlike plasmid-free strains that show indefinite growth potential, repeated vegetative transfer of retroplasmid-containing strains often leads to growth defects and senescence. Presenescent cultures are found to contain variant forms of the plasmids that have cDNA copies of mitochondrial tRNAs or other sequences that derive from mitochondrial DNA (mtDNA; Chiang et al. 1994). These so-called "variant plasmids" may also suffer deletions in non-essential regions (Akins and Lambowitz 1990) or contain duplications of plasmid sequences (Stevenson et al. 2000).

Included among the circular retroplasmid group is the VS plasmid of N. intermedia. It is a small (881 bp), dsDNA that does not encode any large ORFs. The VS plasmid is only found in association with the Varkud retroplasmid and, in experiments in which plasmids were transferred between strains via transient hyphal fusion, the VS plasmid was always transferred along with the Varkud plasmid (Collins and Saville 1990). The VS plasmid appears to replicate via reverse transcription, using the RT encoded by the Varkud plasmid, thus it is also considered to be a satellite plasmid (Kennell et al. 1995). Notably, the VS RNA transcript is a ribozyme and shows highly efficient RNA cleavage and ligation activity (Saville and Collins 1990; Saville and Collins

Table 1 Mitochondrial retroplasmids

Retroplasmid

Type

Host

Size

RT size

Associated

Accession

Refs.

(bp)

(aa)

phenotype

number

Mauriceville

Circular

Neurospora crassa

3581

710

Senescence

NC_001570

Nargang et al. 1984

Varkud

Circular

N. intermedia

3675

710

Senescence

NC_001571

Akins et al. 1988

Maddur-1

Circular

N. intermedia

3614

710

Senescence

AY553872

D'Souza et al. 2005

Maddur-2

Circular

N. intermedia

3774

710

Senescence

AY553873

D'Souza et al. 2005

VS

Satellite

N. intermedia

881

-

-

M327994

Saville and Collins 1990

pThrl

Circular

Trichoderma harzianum

2619

606

-

AF163325

Antal et al. 2002

pFOXCl

Linear

Fusarium oxysporum f. sp.

1900a

n.d.

-

AF005240

Kistler et al. 1996

conglutinans

pFOXC2

Linear

F. oxysporum f. sp. raphani

1905

527

-

AFI 24843

Walther and Kennell 1999

pFOXC3

Linear

F. oxysporum f. sp. matthioli

1836

527

-

AFI 24844

Walther and Kennell 1999

Et2.0L

Linear

Epichloe typhina

2000a

n.d.

-

X57200

Mögen et al. 1991

pRS224-l

Linear

Rhizoctonia solani

4986

887

-

AB035862

Katsura et al. 1997

n.d. not determined a Estimated size

1991; Zamel et al. 2004). A variant form of the Varkud plasmid that contains a portion of the VS plasmid has also been identified (Arganoza and Akins 1995).

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