In situ binding of AT-rich repetitive DNA to the centromeric heterochromatin in polytene chromosomes of chironomids

Histone H1 in the centromeric heterochromatin of Glyptotendipes barbipes larval polytene chromosomes

Immunofluorescent analysis with antibodies against histone H1 failed to detect H1 in the centromeric heterochromatin blocks of the polytene chromosomes of Glyptotendipes barbipes larvae. Centromeric regions were dissected microsurgically and acid-extracted. Electrophoresis in SDS and acid-urea gels revealed a band comigrating with H1 of calf thymus and of Gl. barbipes salivary gland nuclei. ELISA dot assay of the extracted material gave a positive reaction with anti-H1 monoclonal antibodies and with anti-H1 affinity-purified polyclonal antibodies. This shows that the centromeric heterochromatin contains histone H1 but packed in a way which prevents the H1 antigenic determinants from reacting in situ with the specific antibodies.

The fine structure of euchromatin and centromeric heterochromatin in Tenebrio molitor chromosomes

The fine structure of constitutive heterochromatin and euchromatin was compared in electron microscope whole-mount preparations of Tenebrio molitor (Insecta, Coleoptera) spermatocyte nuclei. Tenebrio molitor pachytene chromosomes display extended segments of centromeric heterochromatin and thus are especially suitable for this purpose. When nuclei were incubated in solutions containing different concentrations of NaCl or of MgCl2, two levels of chromatin fine structures were observed in the euchromatic segments: nucleosome fibers (0.1 mM-20 mM NaCl) and supranucleosomal fibers with 28 nm in diameter (40 mM-100 mM NaCl, 0.2 mM-1.0 mM MgCl2). The fine structure in the heterochromatic segments was the same as that in the euchromatic segments in all NaCl concentrations and in MgCl2 concentrations up to 0.4 mM. In higher MgCl2 concentrations the heterochromatin remained more compact than the euchromatin and consisted of 37-nm-thick fibers in 0.6 mM MgCl2 and of 65-nm-thick fibers in 1.0 mM MgCl2. After the 37-nm and the 65-nm fibers had been dispersed in Mg2+-free solutions they could be recondensed by incubation in 0.6 mM and 1.0 mM MgCl2, respectively. It is concluded that a Mg2+-sensitive component of the heterochromatin is responsible for the folding of the nucleosome chain to heterochromatin-specific supranucleosomal structures.

Nucleotide sequence comparisons and functional analysis of yeast centromere DNAs

We determined the nucleotide sequence of DNA segments containing functional centromeres (CEN3 and CEN11) isolated from yeast chromosomes III and XI. The two centromere regions differ in primary nucleotide sequence, but contain structural features in common. Both centromere regions contain an extremely A + T-rich core segment 87-88 bp in length, flanked by two short sequences (14 bp and 11 bp) that are identical in both DNAs. These elements plus one additional 10 bp region of perfect homology are positioned in an almost identical spatial arrangement within the two centromere regions. Significant homologies are also observed among the sequences flanking the high A + T region and various satellite DNA sequences from higher eucaryotes, although no repeated sequences occur near the yeast centromeres. Centromere activity in vivo is maintained on relatively small DNA fragments (627 bp for CEN3 and 858 bp for CEN11), as assayed by mitotic stabilization of autonomously replicating ars plasmids in yeast.