Structural organization of macronuclear chromatin of the ciliate Bursaria truncatella in resting cysts and at excysting.

The structural studies of macronuclear chromatin of the ciliate Bursaria truncatella showed that in resting cysts practically all macronuclear chromatin was arranged in compact clumps 100 to 300 nm in size. By means of negative staining the internal structure of chromatin clumps was revealed; the chromatin clumps were shown to consist of subunits corresponding in size to nucleosomes. Upon incubation in low salt buffer the chromatin clumps decondensed forming loop-shaped chromatin fibres suggesting a similar structural organization of both the chromatin clumps of cysts and the chromatin clumps of grown and differentiated Bursaria truncatella cells [8, 9, 16]. We established that at excysting there occurred natural decondensation of the chromatin clumps resulting in loop-shaped chromatin fibres around the clumps. This in all probability was due to activation of macronuclear chromatin. Sometimes short transcription units of nonribosomal genes (0,2-1,3 micron in contour length) with closely located RNP-fibrils (more than 20 fibrils per 1 micron) were observed on the loops. We suggest that such genes respond to the synthesis of proteins involved in regulation of excysting and/ or subsequent cell growth and differentiation.