Chlamydiaphage Chp2 is a member of the family Microviridae, of which bacteriophage X174 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.Although chlamydiae are widespread bacterial pathogens causing a wide range of illnesses, such as blindness, respiratory infections, and possibly coronary artery disease (23, 24, 26), detailed molecular mechanisms of pathogenesis will remain obscure as long as significant barriers hinder laboratory manipulation. One of these barriers is the lack of a stable gene transfer system. In recent years, five chlamydiaphages belonging to the family Microviridae, of which the prototype is bacteriophage X174, have been isolated (11,14,22,25). The mechanisms and techniques to package X174 in vitro and in vivo are well defined (1, 12). Thus, the microviruses of chlamydiae have the potential of becoming the basis of a genetic transfer system.Critical to this goal is the identification of a chlamydiaphage procapsid, the assembly intermediate into which DNA is packaged. Although the chlamydiaphages are within the same family as X174, the relationship is distant. The family has a deep evolutionary divide between the X174-like phages (coliphages) that infect free-living enterobacteria and the Chp-like phages infecting obligate intracellular parasites and mollicutes, such as the chlamydiae, Bdellovibrio bacteriovorus, and spiroplasma (3). The most striking differences are structural and morphogenetic (3, 5). Structurally, the X174-like phages contain a large pentameric spike protein complex at each vertex of the T ϭ 1 virion. These spikes and their requisite genes are not present in the Chp-like phages, which have elaborate viral coat protein protrusions on the three fold axes of symmetry not seen in the coliphages. Accordingly, structural differences have led to different morphogenetic requirements vis-à-vis scaffolding proteins. The X174-like phages utilize two scaffolding proteins during assembly: an internal and external species, proteins B and D, respectively (10). While it is evident that the Chp-like phages do not encode an external scaffolding pr...
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