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G+C-rich satellite DNA, representing about 19% of total nuclear DNA, was isolated from various tissues of the MOnocotyledonousant, ScitasibeDica, byusing A+-Cs SO4gra-diemnot tedonou is satellite DNA had an unusi hi melting point and a high methylcytosine (m5C) content (-25% of total bases; m5C/cytosine ratio 1L.5) and was localized, by in situ hybridization, in the heterochromatin regions of the chromosomes. Digestion with restriction endonuclease Hae m yielded a series of fragments rangingfrom 35 to several hundred nucleotide pairs. The major fragments, I-IV (35, 50, 59, and 69, nucleotide pairs, respectively), were isolated, and their nucleotide sequences were determined. The dominant fragment I was a highly symmetrical molecule, with a basically palindromic arrangement. This sequence represented the basic unit of Scilla satellite DNA and was tandemly repeated many times, with some base substitutions and multiple successive insertions of the tetranucleotide G-T-C-C. The dinucleotide CpG was the commonest nearest-neighbor sequence. Thin layer chromatography, DNA sequence analysis, and gas chromatography combined with mass spectrometry showed the high m5C content (m5C/Cyt = 2.2 and 2.8, respectively, for fragments II and IEl). Identical cleavage fragments were found in satellite DNAs from two other species of this genus (S. amoena and S. ingridae), which suggests that this constitutively methylated sequence is evolutionarily stable. The sequence arrangement of this plant satellite DNA is compared with those reported for several animal satellite DNAs. Temperature, OCThe nuclear DNA of most, probably all, eukaryotic cells contains fractions of highly reiterated, relatively simple sequences that, in many species, appear as DNA satellites separable from the bulk of the DNA (1). In a variety of animals, the nucleotide sequence arrangement of such satellite DNAs has been elucidated; as a general principle, they contain tandem repeats of a specific sequence unit, which is different in composition and size in different organisms (1-7). Satellite DNAs have also been described (8-11) in several plants that are enriched in heterochromatin. Among the plants that show conspicuous heterochromatin arrays, the monocotyledonous bluebell species, Scilla siberica, has been studied in detail (9,10,(12)(13)(14). The satellite DNA in the heterochromatin of this plant (9, 10) has been analyzed, and the nucleotide sequence of a part of it has been determined. The results show some unusual features ofsatellite DNA organization-multiple tandem arrays of an inverted repeat, multiple insertions of a tetranucleotide, and remarkably high contents of 5-methylcytosine (m5C)

Nuclear membranes were isolated from rat and pig liver by sonication of highly purified nuclear fractions and subsequent removal of adhering nucleoproteins in a high salt medium . The fractions were examined in the electron microscope by both negative staining and thin sectioning techniques and were found to consist of nuclear envelope fragments of widely varying sizes. Nuclear pore complex constituents still could frequently be recognized . The chemical composition of the nuclear membrane fractions was determined and compared with those of microsomal fractions prepared in parallel . For total nuclei as well as for nuclear membranes and microsomes, various enzyme activities were studied . The results indicate that a similarity exists between both fractions of cytomembranes, nuclear envelope, and endoplasmic reticulum, with respect to their RNA : protein ratio and their content of polar and nonpolar lipids . Both membranous fractions had many proteins in common including some membrane-bound enzymes . Activities in Mg-ATPase and the two examined cytochrome reductases were of the same order of magnitude . The content of cytochrome b 5 as well as of P-450 was markedly lower in the nuclear membranes . The nuclear membranes were found to have a higher buoyant density and to be richer in protein . The glucose-6-phosphatase and Na-K-ATPase activities in the nuclear membrane fraction were very low. In the gel electrophoresis, in addition to many common protein bands, some characteristic ones for either microsomal or nuclear membranous material were detected . Significant small amounts of DNA and RNA were found to remain closely associated with the nuclear envelope fragments . Our findings indicate that nuclear and endoplasmic reticulum membranes which are known to be in morphological continuity have, besides a far-reaching similarity, some characteristic differences .