[Structure of the macronucleus chromatin of the ciliate Bursaria truncatella. III. Chromatin organization in resting cysts and during excysting].

Structural organization of macronuclear chromatin of a ciliate Bursaria truncatella was studied electronmicroscopically by means of Miller's technique and negative staining of resting cysts and at excysting. In resting cysts practically all the macronuclear chromatin was shown to be organized into compact chromatin clumps 100-300 nm in size. At excysting a natural decompactization of the chromatin clumps occurred and radial loop-shaped chromatin fibres appeared around the clumps. Sometimes transcription units with a relatively small contour length of transcribed region and with high RNA-polymerase density were observed on the loops. The responsibility of such genes for the synthesis of proteins taking part in regulation of excystment and/or subsequent cell growth and differentiation is discussed.