Organization of transcribed and nontranscribed chromatin in isolated nuclei of Zea mays root cells

Abstract

A slightly modified technique of Miller was applied to nuclei isolated from roots of germinating corn embryos. Under the conditions used, the extranucleolar chromatin was easily spread but the nucleolar chromatin often remained aggregated. The nontranscribed chromatin regions consisted of long deoxyribonucleoprotein (DNP) fibrils with the typical "beads-on-a-string" appearance of nucleosomal arrangement. When the EDTA concentration of the spreading medium was increased, linear arrangement of nucleosomes, 20 nm and 40-50 nm DNP granules were observed on the same DNP fibril. The supranucleosomal granules were also closely packed without any apparent order. Independently of the EDTA concentration, numerous non-nucleolar ribonucleoprotein (RNP) fibrils with a knobby structure were seen laterally bound to DNP fibrils. These knobby RNP fibrils (transcripts) were highly folded and appeared in "bunch-of-grapes" configurations. Each RNP knob measured 23 nm in diameter and the longest of these RNP fibrils were estimated to reach 1.5 micron. The RNP fibrils were often solitary and never exceeded 8 per 10 micron of the DNP axis. Distribution, morphology, and size of these transcripts distinguished them from the nucleolar "Christmas-tree"-like figures, which were rarely observed with our material. These non-nucleolar transcriptional units may correspond to genes coding for the heterodisperse nuclear RNA (hnRNA) and the knobs of 23 nm could be related to informofers or hnRNA particles. Up to now, the visualization of transcriptional units for higher plant tissues have never been reported.