Limited accessibility of chromatin satellite DNA to RNA polymerase from Escherichia coli

Analysis of chromosome replication by a BrdU antibody technique

Chromosome replication was studied without synchronization in human lymphocyte and amniotic cell cultures visualizing very short 5-bromodeoxyuridine (BrdU) pulses by an immunologic technique (BAT). The findings agree in general with those facts known from earlier BrdU staining techniques. The very high sensitivity of BAT was shown to allow the detection of replication in a band where 1 in 200 nucleotides is replaced by BrdU. The main observations are: though the replication patterns after BAT appear strange the bands correspond to those described by the Paris Conference (1971). At the beginning of the S-phase a stepwise onset of replication in only a subset of R-bands is confirmed. There is a considerable difference in the sensitivity between early and late S (SE and SL) for the detection of BrdU pulses. This difference probably reflects a different spatial arrangement of chromatin in R-bands as compared with G-bands below the level of cytogenetic analysis. The use of short pulses did not reveal any additional subdivision of SE or SL. The correspondence between chromosomal bands and replicon clusters is discussed briefly with respect to the different time they need for replication.

In situ nick translation distinguishes between C-band positive regions on mouse chromosomes

In situ nick translation of mouse metaphase chromosomes by non-radioactive detection means and DNase I digestion followed by Giemsa staining were used to analyse the DNase I resistance of two different C-band positive regions. These were the centromeric heterochromatin of acro- and metacentric chromosomes and an interstitial C-band on chromosome 1 of wild mice, IS(HSR;1C5D)1Lub. Whereas the centromeric heterochromatin was clearly resistant to DNase I, the interstitial C-band showed very high DNase I sensitivity. Among centromeric C-bands, the heterochromatin in Robertsonian fusion biarmed chromosomes was more resistant to DNase I action than was the centromeric heterochromatin of the acrocentric chromosomes.

DNase I sensitivity in facultative and constitutive heterochromatin

In situ nick translation allows the detection of DNase I sensitive and insensitive regions in fixed mammalian mitotic chromosomes. We have determined the difference in DNase I sensitivity between the active and inactive X chromosomes in Microtus agrestis (rodent) cells, along both their euchromatic and constitutive heterochromatic regions. In addition, we analysed the DNase I sensitivity of the constitutive heterochromatic regions in mouse chromosomes. In Microtus agrestis female cells the active X chromosome is sensitive to DNase I along its euchromatic region while the inactive X chromosome is insensitive except for an early replicating region at its distal end. The late replicating constitutive heterochromatic regions, however, in both the active and inactive X chromosome are sensitive to DNase I. In mouse cells on the other hand, the constitutive heterochromatin is insensitive to DNase I both in mitotic chromosomes and interphase nuclei.

Initiation of transcription in normal and phosphorylated rat liver nucleosomes

Initiation sites were enumerated in rat liver nucleosomes with Echerichia coli RNA polymerase. One site was present in approx. 3.5 mononucleosomes and in a 1200-base-pair length of polynucleosomes. S-phase nuclei or normal nuclei phosphorylated in vitro showed increased numbers of sites; the elongation rates were unchanged.

In vitro transcription of heat-shock-specific RNA from chromatin of Drosophila melanogaster cells

Polyadenylylated RNA synthesized after heat shock was isolated from polysomes of cultured cells of Drosophila melanogaster and used as template to prepare cDNA. An excess of poly(A)-RNA from heat-shocked cells hybridized to 80% of the cDNA, whereas cytoplasmic poly(A)-RNA from cells grown at 25 degrees could drive only half of the cDNA probe into hybrid. These sequences were removed from the cDNA population by annealing to poly(A)-RNA from cells grown at 25 degrees. The unreacted material represented only heat-shock-induced mRNA sequences, as shown by a second cycle of hybridization. Isolated chromatin was transcribed in vitro at 25 degrees with Escherichia coli RNA polymerase, with mercurated UTP as precursor. RNA transcribed from chromatin that was prepared from cells 1 hr after the temperature was shifted to 37 degrees hybridized with 100-fold faster kinetics to the heat-shock-specific cDNA probe than did RNA transcribed from chromatin of cells grown at 25 degrees. Therefore, heat shock results in a change in chromatin structure recognizable by E. coli RNA polymerase.