ORGANIZATION OF TRANSCRIPTIONALLY INACTIVE CHROMATIN OF THE INTERPHASE MACRONUCLEUS OF THE CILIATE DIDININM NASUTUM

The organization of the transcriptionally inactive chromatin of the in terphase macronucleus of the ciliate Didinium nasutum has been studied in ultrathin sections of fixed cells and in preparations of the chrom atin spread in a low-salt solution (Miller's method). In preparations of weakly dispersed chromatin and in sections, almost all macronuclear chromatin is organized throughout the interphase into electron-dense bodies, 100-200 nm in diameter, connected with one another with one or several fibrils. During spreading, unwinding of chromatin bodies occu rs, and loops formed by nucleosomal fibrils can be observed around a d ense core. There is a structural connection of the artificially decond ensed chromatin with elements of the nuclear envelope. Fully decondens ed chromatin consists of long nucleosome fibrils. At the stage of mid- interphase, agglomerations of granules 19-25 nm in size which were not detected previously in other organisms, occur alternating with agglom erations of nucleosomal chromatin fibres. The two types of agglomerati ons are inter-connected but spatially separated. Continuity of 19-25 n m granules with nucleosome fibrils has been observed, which argues for the DNP nature of these granules. The pattern of spreading obtained c an indicate a cooperative transition of the chromatin bodies (or singl e chromatin loops) from a compact into a decondensed state and back. T he presence of differently compacted chromatin in the macronucleus of Didinium nasutum may reflect differential activity of certain regions of the macronucleus during mid-interphase.