Mycoviruses: Meaning, Types and Replication | Microbiology

In this article we will discuss about:- 1. Meaning of Mycoviruses 2. Types of Mycoviruses 3. Taxonomic Affinity 4. Replication 5. Mycoviruses and Interferon 6. Killer Phenomenon.

Meaning of Mycoviruses:

The viruses associated with fungi are called mycoviruses, and also mycophages. They are often typically latent but some induce symptoms. They are wide spread in all taxonomic groups of fungi. Much less is known about the mycoviruses of the lower fungi.

During 1950s, several disorders in fungi were described and some authors suspected for the involvement of viruses. For the first time Boilings (1962) gave the conclusive evidence of viruses that infected the cultivated mushrooms, Agaricus bisporus causing die back disease. The most characteristics and consistent features of mushroom virus diseases are the loss of crop and the degeneration of mycelium in the compost.

Subsequently mycoviruses from different taxonomic group of fungi were described. So far at least 5,000 fungal species are known to contain mycoviruses. It is interesting to note that most of species of Penicillium and Aspergillus have been found to be infected with viruses, whereas in other genera the viruses have not been found so frequent. Ecologically the mycoviruses are exceptional, possibly unique among the viruses. For their existence they appear to be intracellular, a life style for which they are well fitted.

Types of Mycoviruses:

So far very few mycoviruses have been fully characterized, and most are only the ‘virus like particles’ (VLPs) in electron micrograph of partially purified extracts from the fungus or sometimes from thin section studies. Several different morphological types of VLPs have been observed, some corresponding fairly close with well known viruses of the other host taxa. Some of the examples of mycoviruses are given in Table 18.9.

Table 18.9 : Virus particles reported in fungi.

Some of the isometric particles (105-110 nm diameter) i.e. viruses containing capsid roughly spherical polyhedron, superficially resemble iridoviruses, and some others (about 50 nm diameter) resemble caulimoviruses. Tailed DNA phage particles and paired 20 nm isometric particles of possibly Gemini virus type have also been reported. Most of the virus particles recorded in fungi have been isometric with a genome of several species.

The mycoviruses have a heterogeneous properties with a diameter ranging from 25-50 nm and particle weight from 6-13 x 10⁶ Dalton. They possess 1-8 segments of dsRNA with a total molecular weight of 2-8.5 x 10⁶ Dalton.

All the examined samples had only a single capsid protein but of varying molecular weight from 25 to 130 x 10³ Dalton in different viruses. However, the mycroviruses with more than one capsid polypeptide have also been reported in the purified preparations of many potyviruses, the other viruses of higher plants and in Aspergillus foetidus virus- S (Afv-s).

Taxonomic Affinity:

The mycoviruses poses a frustrating taxonomic problem. However, no serological relationship have been detected between any myco-virus and morphologically similar viruses in higher plants.

Similarly, no mycoviruses could be demonstrated to infect higher plants. In addition, the isometric dsRNA mycoviruses have no clear affinity with dsRNA viruses of the other host taxa or taxonomic unity among the viruses with dsRNA genome (Table 18.10).

It could be noted that the dsRNA mycoviruses have evolved on more than one occasion and that the dsRNA genome reflects adaptation to the conditions within the fungal cell. Therefore, the members of the International Committee for Virus Taxonomy resisted to the temptation to set up a large taxonomic group to accomodate all dsRNA mycoviruses.

Two such groups now has been designated:

(i) The Penicillium chrysogenum virus group, and

(ii) The P. stoloniferum virus – S (PsV-s) group.

The member viruses within each of these groups are serologically related.

Replication of Mycoviruses:

Buck (1979, 1980) has reviewed the replication of mycoviruses inside the fungal cell. However, the major difficulty to understand the replication strategies of the dsRNA mycoviruses is the lack of methods to get in vitro infection of a healthy fungal cell by free virus preparation. Hopefully, the dsRNA seems unable to act as mRNA in the fungal cell and may sometimes inhibit protein synthesis.

Buck (1980) has reported the host cell enzymes capable of transcribing the ssRNA and dsRNA in vitro and probably dsRNA in vivo. Highly specific virus coded RNA polymerases are necessary for effective in vivo transcription and replication of dsRNA. Such polymerase has been demonstrated in a number of dsRNA mycoviruses. Probably, the polymerase remain confined within the virus particle during the replicative cycle of mycoviruses.

Within undivided genome of vims L of Saccharomyces cervisiae, two types of RNA polymerase activity has been detected: ds→ssRNA (transcriptase) and ss→dsRNA. These activity permits viral L dsRNA to replicate synchronously as occurs in reovirus with several differences.

In AfV-S the transcriptase activity has been noted and possibly replication occurs through semiconservative strand displacement. This differs fully from semi-conservative reovirus system.

Also, PsV-S shows replicase activity in vitro giving rise to dsRNA progeny molecules that remain encapsulated. This type of semiconservative replication and strand displace­ment as found in adenovirus DNA would suggest a synchronous DNA. In vivo studies suggest that replication of PsV-S dsRNA may in-fact be asynchronous.

Mycoviruses and Interferon:

Until 1960s, an extensive study was done on the induction of interferon (antiviral chemical compound) by culture filtrates and cell extracts of some isolates of P.stoloniferum and P.funiculosum in mice. Viral dsRNA was the inducer of interferon when viral particles were injected into the animals.

The dsRNA prepared from the virus induced interferon production in mice as did extracts from the culture filterate containing both the viral particles and RNA. Free RNA was apparently more potential stimulant of interferon production than the virus particles. Its effects were more rapid though less lasting than whole virus.

This aroused the prospects of clinical and veterinary sciences. A number of preparative processes for industrial production of myco-virus extracts were patented. Some success was also achieved in suppressing the virus infection such as common cold. Soon it was reported that it has several serious side-effects.

However, it is still unclear that these effects are derived from toxicity of breakdown products of the injected materials or are an integral part of the interferon response. Consequently, funding of projects got ceased. This aspect has been well reviewed by Klein Schmidt (1979) and Dubey (1985).

The Killer Phenomenon of Mycoviruses:

In recent years, much emphasis has been laid on the production of ‘killer toxin’ and myco-virus dsRNA segment. Some strains of S.cerevisiae and Ustilago maydis (com smut fungus) secrete extracellular toxins that either kill or suppress the growth of same or related fungal species but each killer strain is immune to its own toxin.

The most sensitive race of S.cerevisiae contains the viral particles of 40 nm diameter with a single dsRNA designated as L. The Killer strains also contain a dsRNA of molecular weight of about 1.1 – 1.4 × 10⁶ designated as M. The coat proteins of both are indistinguishable serologically or in electrophoresis.

The M dsRNA encodes the labile glycoprotein killer strain (confirmed by in vivo translation), and is believed to determine the immunity factor. The M is found only in cells containing the L dsRNA. The L is supposed to encode both RNA polymerase and the coat protein for L and M RNAs.