Chlamydiaphage Chp2, a skeleton in the phiX174 closet: scaffolding protein and procapsid identification.

Chlamydiaphage Chp2 is a member of the family Microviridae, of which bacteriophage phiX174 is the type species. Although grouped in the same family, the relationship between the Microviridae coliphages and the Chp2-like viruses, which infect obligate intracellular parasitic bacteria, is quite distant, with major differences in structural protein content and scaffolding protein dependence. To investigate the morphogenesis of Chp2, large particles were isolated from infected Chlamydophila abortus by equilibrium and rate zonal sedimentation. A monoclonal antibody that recognizes only assembled viral coat proteins was used in these detection assays. Thus, the detected particles represent virions and/or postcapsid formation assembly intermediates. Two distinct particle types were detected, differing in both protein and DNA content. Filled particles lacked VP3, the putative internal scaffolding protein, whereas empty particles contained this protein. These results indicate that VP3 is a scaffolding protein and that the isolated VP3-containing particles most likely represent Chp2 procapsids.

Microviridae, a family divided: isolation, characterization, and genome sequence of phiMH2K, a bacteriophage of the obligate intracellular parasitic bacterium Bdellovibrio bacteriovorus.

A novel single-stranded DNA phage, phiMH2K, of Bdellovibrio bacteriovorus was isolated, characterized, and sequenced. This phage is a member of the Microviridae, a family typified by bacteriophage phiX174. Although B. bacteriovorus and Escherichia coli are both classified as proteobacteria, phiMH2K is only distantly related to phiX174. Instead, phiMH2K exhibits an extremely close relationship to the Microviridae of Chlamydia in both genome organization and encoded proteins. Unlike the double-stranded DNA bacteriophages, for which a wide spectrum of diversity has been observed, the single-stranded icosahedral bacteriophages appear to fall into two distinct subfamilies. These observations suggest that the mechanisms driving single-stranded DNA bacteriophage evolution are inherently different from those driving the evolution of the double-stranded bacteriophages.