Incorporation of argyrophilic proteins into the nucleolus‐associated chromatin caused by exposure to actinomycin D

Abstract

The nucleolus-associated chromatin (NAC) became argyrophilic when the root tips of Vicia faba were treated with 0.2 microgram/ml actinomycin D (AMD) for 15 hr at 22 degrees C. To determine how the argyrophilic NAC is formed, detailed observations were carried out on thin sections following silver staining and ribonucleoprotein (RNP) preferential staining. In a few instances, thick channels with loosely packed fibrillar material (light fibrillar area) were seen meandering throughout the nucleoli. Large granules 280-330 A in diameter (300 A granules), which responded positively to both RNP preferential staining and silver staining, were sometimes present within the light fibrillar area. Some fragments of NAC were also seen in the light fibrillar area and, interestingly, they responded positively to silver even though they were bleached by the RNP preferential staining. Most nucleoli showed a segregation of two major components, the fibrillar and granular components. The nucleolar vacuoles were localized strictly within the fibrillar component and usually accompanied the light fibrillar area or, alternatively, contained a number of 300 A granules. Another category was characterized by the presence of poorly developed vacuoles in which the 300 A granules were still found, and the spherical argyrophilic NAC associated with the surface of the nucleolus. The present observations suggest that AMD induces the DNA-containing structure engaged in ribosomal RNA transcription to condense, and during this process some argyrophilic proteins are incorporated into the chromosomal structure thereby causing the NAC to become argyrophilic. In this context, the NAC is assumed to contain ribosomal RNA genes.