Spv1 and Spv4, Spiroplasma Viruses With Circular, Single-Stranded Dna Genomes, and Their Contribution to The Molecular Biology of Spiroplasmas

Abstract

Publisher Summary This chapter focuses on SpV1, and SpV4, two circular single-stranded DNA viruses. SpVl has been classified in the Inoviridae family, genus Plectrovirus. SpV4 belongs to the Microviridae family, genus Spiromicrouirus. SpVl and Spy4 are spiroplasma viruses with circular singlestranded DNA genomes. The SpV4 genome was the first self-replicating element among members of the class Mollicutes to be sequenced, and revealed that gene organization, transcription, and translation were similar to those in eubacteria. These results confirmed that members of the class Mollicutes were not primitive organisms but have evolved from ancestors of gram-positive bacteria with low G + C DNA, such as certain clostridia. Identification of the SpV4 capsid protein gene revealed that in spiroplasmas, like in mycoplasmas, UGA is a tryptophan codon and not a stop signal for translation, and that, because of low G + C, codons synonymous with A or T in the wobble position are preferred over those with G or C. One of the most unexpected findings is the presence of SpVl DNA sequences in many regions of the spiroplasmal chromosome. Replication of the single-stranded circular SpVl DNA involves doublestranded replicative form (RF) DNA. Insertion of SpVl sequences into the host chromosome requires linearization of the circular RF. The site of linearization has been identified for two inserted viral sequences.