MORPHOLOGY AND INTRACELLULAR LOCALIZATION OF A BACILLIFORM LATENT VIRUS IN SWEET CLOVER

In two species of symptomless sweet clover (Melilotis alba Desr. and M. officinalis (L.) Lam.), bacilliform particles were observed. The viral nature of these particles was established through transmission by tissue union and by morphological characteristics; thereupon, they were designated as Melilotis latent virus (MLV). Bacilliform particles occurred as single units or monomers 300-350 mμ long and as dimers 600-300 mμ long. In addition, bullet-shaped particles 230-300 mμ long were prevalent in negatively stained preparations. These latter particles may originate from breakage of dimers into units of equal length. All particles were ca. 80 mμ in diameter and consisted of an outer coat 70 Å thick, an inner tubular component 600 Å wide with a 100 Å wide axial core. The wall of the inner component was 100-120 Å thick and contained transverse striations with a 45-50 Å periodicity. Most virions occurred within the perinuclear space. The inner component of the virion was observed within the nucleus. Mature virions, containing both the inner component and outer coat, were present within the perinuclear space, and the outer coat was frequently continuous with the inner membrane of the nuclear envelope. It thus seems likely that the complete virion was assembled as the inner component was extruded from the nucleus into the perinuclear space through the process of synhymenosis. Mature virions also occurred in the cytoplasm within membrane-bound vesicles, possibly arising from delamination of the nuclear envelope. © 1969 Academic Press, Inc.