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The rate of introduction of neutral mutations is lower in man than in other primates, including the chimpanzee. This species is generally regarded as our closest relative among the great apes. We present here an analysis of sequences of X chromosomal alphoid repetitive DNA from man and the great apes, which supports the closer relationship between man and chimpanzee and indicates a considerably increased rate of recombination in the human repeat DNA. These results indicate that the 'molecular clock' is running more quickly in man.

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Centromeric alphoid DNA in primates represents a class of evolving repeat DNA. In humans, chromosomes 13 and 21 share one subfamily of alphoid DNA while chromosomes 14 and 22 share another subfamily. We show that similar pairwise homogenizations occur in the chimpanzee (Pan troglodytes), where chromosomes 14 and 22, homologous to human chromosomes 13 and 21, share one partially homogenized alphoid DNA subfamily and chromosomes 15 and 23, homologous to human chromosomes 14 and 22, share another extensively homogenized subfamily. Such a pattern of homogenization presumably predates speciation 3-10 million years ago. However, the alphoid DNA on these human and chimpanzee chromosomes is not orthologous but originates from two evolutionarily different repeat families. It follows that dramatic sequence evolution has occurred in a concerted fashion among the chromosomes in one or both species during or after separation.

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The nucleotide sequence of members of an alpha-repeat subfamily shared by human chromosomes 14 and 22 is presented. This subfamily is organized into a higher-order repeat unit composed of a tandem repetition of an ordered array of four related but distinct 340-bp repeat dimers. An analogous situation has been described for a related but distinct subfamily shared by chromosomes 13 and 21. These two subfamilies were further shown not to be present on the homologous chimpanzee chromosomes and therefore must have arisen by rearrangement of the human genome after separation of the two species. The sequence homology between the 13/21 and the 14/22 subfamilies is about 85%. The 14/22 subfamily represents the only major alphoid DNA species on these two chromosomes and is not present elsewhere in the human genome. Fluorescent in situ hybridizations show that sequences from the 13/21 and 14/22 subfamilies can be used as specific markers for their respective chromosomes.

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The alphoid repeat DNA on chimpanzee chromosome 22 was compared with alphoid repeat DNA on its human homologue, chromosome 21. Hybridization of different alphoid probes under various conditions of stringency show that the alphoid repeats of chimpanzee chromosome 22 are not closely related to those of human chromosome 21. Sequence analysis of cloned dimer and tetramer EcoRI fragments from chimpanzee chromosome 22 confirm the low overall level of homology, but reveal the presence of several nucleotide changes which are exclusive to the chromosome 21 subfamily of human alphoid DNA. Southern blot analysis of alphoid repeat DNA on the chimpanzee X chromosome suggests this subfamily has been strongly conserved during and since the separation of chimpanzee and man although the two subfamilies can be distinguished on the basis of Taq I restriction fragments.

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The organization of alphoid repeated sequences on human nucleolus-organizing (NOR) chromosomes 13, 21, and 22 has been investigated. Analysis of hybridization of alphoid DNA probes to Southern transfers of restriction enzyme-digested DNA fragments from hybrid cells containing single human chromosomes shows that chromosomes 13 and 21 share one subfamily of alphoid repeats, whereas a different subfamily may be held in common by chromosomes 13 and 22. The sequences of cloned 680-base-pair EcoRI fragments of the alphoid DNA from chromosomes 13 and 21 show that the basic unit of this subfamily is indistinguishable on each chromosome. The sequence of cloned 1020-base-pair Xba I fragments from chromosome 22 is related to, but distinguishable from, that of the 680-base-pair EcoRI alphoid subfamily of chromosomes 13 and 21. These results suggest that, at some point after they originated and were homogenized, different subfamilies of alphoid sequences must have exchanged between chromosomes 13 and 21 and separately between chromosomes 13 and 22.

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Nucleotide sequence data of about 20 X 10(3) base-pairs of the human tandemly repeated alphoid DNA are presented. The DNA sequences were determined from 45 clones containing EcoRI fragments of alphoid DNA isolated from total genomic DNA. Thirty of the clones contained a complete 340 base-pair dimer unit of the repeat. The remaining clones contained alphoid DNA with fragment lengths of 311, 296, 232, 170 and 108 base-pairs. The sequences obtained were compared with an average alphoid DNA sequence determined by Wu & Manuelidis (1980). The divergences ranged from 0.6 to 24.6% nucleotide changes for the first monomer and from 0 to 17.8% for the second monomer of the repeat. On the basis of identical nucleotide changes at corresponding positions, the individual repeat units could be shown to belong to one of several distinct subfamilies. The number of nucleotide changes defining a subfamily generally constitutes the majority of nucleotide changes found in a member of that subfamily. From an evaluation of the proportion of the total amount of alphoid DNA, which is represented by the clones studied, it is estimated that the number of subfamilies of this repeat may be equal to or exceed the number of chromosomes. The expected presence of only one or a few distinct subfamilies on individual chromosomes is supported by the study, also presented, of the nucleotide sequence of 17 cloned fragments of alphoid repetitive DNA from chromosome 7. These chromosome-specific repeats all contain the characteristic pattern of 36 common nucleotide changes that defines one of the subfamilies described. A unique restriction endonuclease (NlaIII) cleavage site present in this subfamily may be useful as a genetic marker of this chromosome. A family member of the interspersed Alu repetitive DNA was also isolated and sequenced. This Alu repeat has been inserted into the human alphoid repetitive DNA, in the same way as the insertion of an Alu repeat into the African green monkey alphoid DNA.

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Sequences analogous to the intragenic bipartite RNA polymerase III promoter regions in tRNA genes, are described in mammalian Alu-type repetitive DNA, LTR of retroviruses and control regions of papovaviruses. The RNA polymerase III control regions in some of these DNA elements are present in enhancer sequences which are known to increase the transcription of RNA polymerase II transcribed genes. It is suggested that the host and tissue specificity of the enhancer elements may be explained by differences in the RNA polymerase III control sequences. The RNA polymerase III promoters may thus participate in regulation of cell differentiation and transcription of tissue specific genes.

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We have described a characteristic substructure of mitotic chromosomes, the chromosomal unit fibre, with lengths about five times the length of the corresponding metaphase chromosomes and a uniform diameter of 0.4 micrometer. In order to study the relationship of chromosome banding to chromosome compaction, methods have been devised to obtain banding patterns on chromosomal unit fibres, similar to G-band patterns of intact mitotic chromosomes. The total number of bands plus interbands per haploid human karyotype is estimated at about 3000. The banding pattern of chromosomal unit fibres indicates a certain resemblance to the normal G-banding pattern of human chromosomes even if the details indicate a short-range random distribution.

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The length of 44 silver-stained human autosomal pachytene complements was shown to vary from about 300 micrometer to at least 535 micrometer. The lengths of the individual 22 autosomal chromosomes of eight complements representing this interval was measured and the relative lengths calculated. For most of the chromosomes a 1:1 relationship was found between the relative length and the corresponding relative DNA content (Mendelsohn et al. 1973). For some of the chromosomes this ratio seemed to deviate systematically from the 1:1 ratio. The data indicate a different organization of certain stages of the meiotic and the mitotic chromosome (Bak et al. 1979).

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A structural component of mitotic chromosomes that partially explains the compaction of DNA within mitotic chromosomes is suggested on the basis of the occurrence of long, regular cylindrical structures in preparations of isolated human chromosomes. These structures, "unit fibers", of a rather constant diameter of about 4,000 A have been postulated to be formed by coiling of the 250-300 A solenoid chromatin fiber that itself is formed by coiling of the 100 A string of nucleosome fiber. The human chromatid would thus be composed by a hierarchy of helices with contraction ratios for DNA at each level of coiling of 7 (string of nucleosomes), 5 (solenoid) and 40 (4,000 A "unit fiber" or "super-solenoid") which results in an overall contraction ratio for DNA in the "unit fiber" structures of about 1,400, which is approximately 5-fold less than the final contraction of DNA in intact chromatids of condensed metaphase chromosomes. The present report concerns more detailed studies with respect to the dimensions and cytochemical properties of the "unit fiber" structures observed in preparations of isolated human mitotic chromosomes that provide direct and indirect evidence in support of their "super-solenoid" structure and relate to known properties of human mitotic chromosomes.

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Chromosomal unit fibers consisting of long, regular fibers of about 0.40 micron diameter were obtained from disintegrated, isolated chromosomes of two Drosophila melanogaster cell lines. In one cell line with an essentially normal karyotype, three clearly defined size classes of 15,13, and 11 micron length were observed corresponding to the three larger chromosomes of Drosophila. In a cell line carrying an additional translocation between the two largest chromosomes a 19 micron fiber derived from the translocation chromosome was observed. Direct determinations of the DNA content per micron length of Drosophila unit fibers show that DNA is contracted by a factor of about 1400x in agreement with calculations based on the length of the unit fibers and the known DNA content of the individual Drosophila chromosomes. These findings support our previously proposed model for the unit fiber sub-structure of chromosomes as being derived by a hierarchy of coiling with the corresponding contraction ratios being 7 (100 A string of nucleosomes), 5 to 6 (250-300 A thick nucleohistone fiber), and about 40 (unit fiber), resulting in a total contraction of DNA in unit fibers in the order of 1400x.

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From observations on the partial disintegration of isolated human metaphase chromosomes we propose that human metaphase chromatids have a rather simple organization based on the folding and coiling of a long, regular, hollow cylindrical structure with a diameter of about 4000 A. This cylindrical structure, the unit fiber, is postulated to be a super-solenoid formed by the coiling of a 300 A solenoid, itself composed by coiling the basic string of nucleosomes. The structure of a human chromatid would thus be a hierarchy of helices, the contraction ratio of each coil, in ascending order of size, being approximately 7, 6, 40, and 5. This model appears to explain the estimated mass/unit length and accounts for many of the known features of human mitotic chromatids.

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The patterns of drug resistance and the frequency of conjugative R plasmids in intestinal Escherichia coli from 88 patients treated for a skin disease (acne vulgaris) with low oral doses of tetracycline are reported. The proportion of patients with resistant bacteria was progressively greater in patients who received tetracycline for 1 week, 4 weeks, or longer (from 50 to 88%). No multiply drug-resistant bacteria were detected before treatment or after 1 week of treatment. After more than 4 weeks of treatment, multiply drug-resistant E. coli were isolated from about 50% of the patients. The origin and selection of R plasmid-determined multiple drug resistance are discussed.

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Extrachromosomal deoxyribonucleic acid (DNA) from 24 different R factor-harboring Enterobacteriaceae was isolated and characterized by analytical ultracentrifugation and electron microscopy. The R factors represented 15 different patterns of transferable drug resistance found in enterobacteria from an enclosed geographic area. All of the strains contained extrachromosomal, circular DNA molecules within the range of 0.4 to 52 mum. More than one size class of circular DNA molecules was observed in the majority of the extrachromosomal DNA preparations. The buoyant density of the extrachromosomal DNA ranged from 1.700 to 1.720 g/cm3. The majority of the bacteria contained extrachromosomal DNAs of various densities. Three-fourths of the R factors were classified as fi+. The investigation illustrates the extensive variability in the physical characteristics of plasmid DNA from R factor-harboring strains.

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Drug resistance to 8 different antibiotics in Enterobacteriaceae isolated from different hospitals and two groups of general practitioners was studied. Escherichia coli dominated among the 632 strains investigated. Drug resistance was found in 62% of the 512 hospital strains and in 38% of the 120 strains from general practice. Multiple resistance was common especially in strains from hospital. R factors was found in 23% of the 317 drug-resistant strains from hospital and in 11% of the 46 drug-resistant strains from general practice. Resistance to streptomycin, sulphonamide and tetracycline either alone or in combinations were the most common traits transferred.

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