Evolution and Evolutionary Studies

In the past decade, two approaches have been taken to investigate Microviridae evolution. In one approach, pioneered by the Drs. J. J. Bull and H. A. Wichman and colleagues, viruses are placed under selective conditions, either high temperature or host variations, and grown for numerous generations in a chemostat (23,24,31,69,137). At various time intervals, individual genomes are sequenced. Therefore the appearance and disappearance of beneficial mutations can be monitored. Of course, host adaptation experiments have identified mutations that increase attachment and penetration efficiency. But mutations that affect intracel-lular tropism, more specifically mutations in gene A, were also uncovered. Protein A adaptation would be needed to optimize its interaction with the host cell rep protein, which functions as a host cell helicase during stage II and stage III replication (41). The high temperature selection yielded several types of mutations. In addition to mutations that appear to affect both intracellular and extracellular interactions, which may be needed to stabilize macromolecular interactions at higher temperatures, many mutations affecting morphogenesis and/or the stability of the procapsid were isolated. Considering the metastable properties of assembly intermediates, these results are not surprising.

In both selections, mutations in genetic regulatory sequences were also recovered. These mutations most likely optimize the relative level of viral proteins synthesized under the experimental conditions. However, it should not be assumed that these mutations lead to elevations in transcription, transcript stability, or translation. Adaptation may involve maintaining an optimal balance of viral components (48, 125). For example, the expression of a relatively stable protein under selection conditions may be downregulated while the expression of less stable proteins may be upregu-lated. Some recurrently recovered neutral mutations may also be acting on this level, changing the intracellular level of the encoded protein by codon usage. However, neutral mutations may also be acting on a structural level by altering a genome secondary structure. As discussed above, the interplay between a genome's secondary structure and its interactions with the capsid's inner surface may affect the final stages of virion morphogenesis and can create capsid surface distortions that reduce host cell attachment. In several the selections, a deletion in the F-J intercistronic group was recovered. This deletion was also isolated as a mutation that elevates the rate of host cell attachment and penetration (74). In addition, it acts as a second-site suppressor of mutations that affect the interface between the genome and the inner surface of the viral capsid (S. Hafenstein and B. A. Fane, unpublished data).

The second area of evolutionary research has focused on the isolation of novel members of the family. As stated by Hendrix et al. (66, 67), the prevalence of double-stranded DNA phages and prophages—cryptic, defective, and replication competent—creates an enormous pool of evolutionary material which can be horizontally exchanged, otherwise known as the moron accretion hypothesis. Consequently, a mosaic spectrum of related phage species has arisen. In contrast, the members of the Microviridae appear to fall into two distinct and rather distantly related subfamilies. Protein homologies between the two subfamilies are approximately 20% or less (table 11-2), a typical value when comparing the most distantly related members of either the lambda or T4-like groups (66, 67, 130). However unlike tailed dsDNA families, no mosaic species that bridge the evolutionary chasms have been isolated. The members of the fX174 subfamily were isolated from g-proteobacteria,

Table 11-2 Amino Acid Identities of f MH2K Gene Products with Chp1, Chp2, SpV4, and 0X174, and Amino Acid Homologies

Percent amino acid identity

Genea -

product Chp2 Chp1 SpV4 Chp2/Chp1b f X174-likec

Percent amino acid identity

Genea -

product Chp2 Chp1 SpV4 Chp2/Chp1b f X174-likec

0 0

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