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In a previous paper in this journal [Djondjurov, L., Ivanova, E. and Tsanev, R. (1979) Eur. J. Biochem. 97, 133–139], we showed that a nuclear fraction released from chromatin under a mild nuclease digestion contained an increased amount of hnRNA and the bulk of nonhistone proteins with a high metabolic rate. The present investigation has revealed that the nonhistone proteins of this fraction could be divided into three distinct metabolic groups. The first group consists of proteins with a fast turnover rate (mean half-life 30 min) which migrate into chromatin immediately after their synthesis. These proteins are predominantly acid-soluble and have relatively high molecular weights. The second group includes proteins which migrate to the nucleus more slowly and metabolize with a moderate turnover rate (mean half-life 5 h). The third group contains proteins with a more conservative metabolic behaviour. In experiments with actinomycin D it was found that the bulk of the nonhistone proteins of this fraction are not real components of the chromatin but belong to the protein moiety of heterogeneous nuclear ribonucleoprotein particles associated with chromatin. [ABSTRACT FROM AUTHOR]

The chromatin structure of the macronuclear genes encoding ribosomal proteins S25 and L1 in the ciliated protozoan Tetrahymena thermophila was analyzed. Using the indirect end-labelling technique, DNase-I-hypersensitive regions were located in the promoter regions as well as in the 3' regions of the genes. The DNase-I-hypersensitive regions were present in chromatin of exponentially growing cells, where the rate of ribosomal-protein gene transcription is high, and in chromatin from starved cells, where transcription of ribosomal-protein genes is severely depressed. Micrococcalnuclease-digestion experiments revealed that the promoter regions of the S25 gene and the L1 gene are devoid of nucleosomes in exponentially growing cells. In starved cells, no nucleosomal organisation of the promoter region of the L1 gene could be detected, whereas nucleosomal structures were discernible in the promoter region of the S25 gene. A conspicuous polypurine sequence motif, AARGGGAAA, is present within or adjacent to the DNase-I-hypersensitive regions in the promoter of the S25 and the L1 gene, and interestingly, the same motif is found also in the promoter regions of the genes encoding ribosomal proteins L21 and L37. [ABSTRACT FROM AUTHOR]

The extrachromosomal DNA coding for the ribosomal RNA precursor in Tetrahymena contains a transcribed region with a size of 6 x 10[sup3] base pairs plus non-transcribed central and distal spacers. In the present study the chromatin structure of the transcribed region and the terminal spacer have been compared. Micrococcal nuclease and DNase I were used to investigate the nucleosomal and the higher order structures. The specific DNA fragments were visualized by gel electrophoresis, Southern blotting onto nitrocellulose sheets and hybridization with specific [sup32]P-labelled RNA probes. Investigations of the cleavage patterns demonstrate the presence of a defined nucleosomal structure in the non-transcribed region, while there is no indication of a nucleosomal pattern in the transcribed region. Specific regions on the r-chromatin are hypersensitive to DNase I. The first cleavage occurs in the non-transcribed central spacer region, while the second cleavage takes place in a region near the 3[sup'] end. The hypersensitivity of the central part of r-chromatin is also found by autodigestion in isolated nucleoli. [ABSTRACT FROM AUTHOR]