Single-stranded DNA Mycoplasmaviruses

Abstract

We have been studying the replication of group-1 mycoplasmaviruses. Mycoplasmas are the smallest free-living prokaryotes (see review by Maniloff and Morowitz 1972). The virus host is Acholeplasma laidlawii, a cell that has a genome of 1 × 10⁹ daltons and, like other mycoplasmas, is bounded by only a 7-nm plasma membrane. The group-1-virus isolate that has been used in most studies, MVL51, is a bullet-shaped particle containing a molecule of circular single-stranded (SS) DNA of about 1.5 × 10⁶ daltons. Virus infection is not lytic: infected cells continue to grow and extrude progeny viruses (Liss and Maniloff 1973a).

The biology of this virus and of the two groups of double-stranded DNA mycoplasmaviruses has been reviewed recently (Maniloff et al. 1977). We will consider here experimental data describing the molecular aspects of MVL51 structure and replication.

STRUCTURE
Morphology
Virions of MVL51 are unenveloped, bullet-shaped particles (Fig. 1) about 14 nm by 80 nm in size (literature reviewed by Maniloff et al. 1977). Optical analysis of electron micrographs of negatively stained virions has shown that the particles have helical symmetry (Bruce et al. 1972; Liss and Maniloff 1973a). The optical transforms have a near-meridional reflection at 4.8 nm, so the distance between helix turns is 4.8 nm. A model for MVL51 has been proposed by Bruce et al. (1972) based on the diffraction data. This model is a T = 1 icosahedron with a diameter of 14 nm, cut across a twofold axis, with structure units added to form a helix. This...