Localization of DNA phosphorus in Tipula iridescent virus by oxygen plasma microincineration. I. Electron microscopy of ash structures.

Tipula iridescent virus (TIV) is a relatively large particle containing about 15% DNA. As shown elsewhere by X-ray microanalysis (Thomas RS, Corlett M: J Histochem Cytochem 29:394, 1981), low-temperature oxygen plasma microincineration of the virus produces a stable phosphorus oxide ash representing this DNA nearly quantitatively. Transmission electron microscopy (TEM) of entire particles after plasma incineration shows the ash confined to the viral cores, confirming the previously known general location of the nucleic acid. Examination of ultrathin-sectioned virus crystals after plasma etching or ashing shows, on a still finer scale, that the DNA is probably confined to a shell structure within the core. A fine trace of ash from the capsid, seen in some preparations, may represent a phospholipid internal membrane known to be present. The possibilities of ash pattern artifacts are discussed. Heating experiments show that the ash patterns (and native virus particles) evaporate completely at high temperature, consistent with their presumed polyphosphoric acid composition. A heat-stable ash could be formed, however, when the viral DNA became accidentally stained with iron from the steel TEM grids used--a noteworthy artifact. The present work suggests some future possibilities of the plasma microincineration technique. In particular, the ability to see directly the fine distribution of mineral concentrations in ash patterns with the full resolution of TEM should be a powerful adjunct to increase effectively the sensitivity and resolution of X-ray microanalysis of mineral constituents in biological specimens.