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Currently Alzheimer's disease, which affects more than 20 million people worldwide, can only be definitely diagnosed by histological examination of brain tissue obtained at autopsy or biopsy. There is a great need for an early, noninvasive, sensitive, and easily administered diagnostic test of Alzheimer's disease. Here it is reported that patients diagnosed with probable Alzheimer's disease by standard clinical criteria exhibited a marked hypersensitivity in their pupil dilation response to a cholinergic antagonist, tropicamide, placed in their eyes. It was possible to distinguish 18 of 19 individuals (95%) either clinically diagnosed with Alzheimer's disease or classified as suspect Alzheimer's individuals by neuropsychological screening from 30 of 32 normal elderly controls (94%).

During the development of Xenopus oocytes there is a special DNA synthesis that leads to a thousandfold amplification of the genes that code for ribosomal RNA. We have used the electron microscope to study this process. Our primary observation is the presence of ribosomal DNA in rolling-circle intermediates at the time of amplification. We believe that these intermediates are involved in the amplification process, and as such offer the first example of the involvement of a rolling circle in the replication of eukaryotic DNA.Gene Amplification. During oogenesis in many organisms a specific replication of the genes that code for ribosomal RNA occurs (for a review, see ref. 1). The purpose of this well-controlled and selective DNA synthesis is apparently to provide additional template for the massive ribosomal RNA synthesis that occurs during the development of the oocyte. The product RNA is packaged into ribosomes which are stored until they are used for the extensive protein synthesis of embryogenesis.Amplification of ribosomal DNA has been best documented in the studies of Brown and Dawid (2) and Gall (3) with Xenopus laevis. In this species of South African toad, there is about a thousandfold amplification of the ribosomal RNA genes. The product DNA appears in multiple nucleoli which, in contrast to the two nucleoli of somatic cells, are found in the nucleoplasm unattached to the chromosomes (1, 4-7). The amplified DNA, as it serves as a template for ribosomal RNA transcription, has been observed with the electron microscope by Miller and Beatty (8).Fine Structure of Amplified Genes. The structure of the Xenopus laevis genes that code for 18S and 28S ribosomal RNA (rDNA) has been extensively studied in the laboratories of Brown, Birnstiel, and Miller. These studies show that ribosomal DNA is a repetitive DNA in which each repeating sequence has a molecular mass of about 8 million daltons, or about 13,000 base pairs (9). Each repeat of ribosomal DNA is composed of two regions. One of these regions contains the nucleotide sequences coding for 18S and 28S rRNA (10,11) and is transcribed into a 40S piece of RNA that serves as a common precursor (12). The remainder of the rRNA gene consists of a spacer region that is not transcribed and whose function is unknown (8,10,11,13). A diploid Xenopus somatic cell contains about 900 copies of the rRNA gene (10, 11). Some, perhaps all, of these rRNA genes are arranged in a tandem array, head to tail, along the DNA (9, 14, 15). The DNA product of the selective amplification of ribosomal RNA genes is very similar in organization to somatic ribosomal DNA (9, 13).The mechanism by which ribosomal RNA genes are selectively amplified is an unsolved problem. We have used the electron microscope to search for intermediates in the amplification process. Our results indicate that amplification of ribosomal RNA genes involves a rolling-circle intermediate. METHODS Isolation of ribosomal DNA during amplificationXenopus laevis tadpoles were obtained from the Amphibian Facility of ...

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There is reason to believe' that host-cell enzymes catalyze several of the steps in the replication of the single-stranded DNA of the virus 4bX174, particularly the conversion of the single-stranded DNA to the double-stranded replicative form (RF)l and, perhaps, the subsequent replication of the RF. Hence one could expect to find bacterial mutants in which the 4X DNA cannot be replicated. Such mutants would more clearly define host-controlled steps in the viral life cycle. In addition, one might be able to use the viral DNA as a probe of the fate of nucleic acids in the mutant cell; that is, it should be possible to understand any concomitantly occurring deficiency in cellular nucleic acid metabolism in terms of the experimentally analyzable defect in 4X DNA replication.Because of the potentially informative nature of mutants with altered patterns of DNA metabolism, and because of our interest in 4X replication, we sought mutants of a 4IX-sensitive strain that would adsorb but not replicate the virus. We report here the initial characterization of a bacterial mutant (called REP-) that does not allow the replication of 4X RF and additionally has a reduced capacity for bacterial recombination. It is representative of a heretofore undescribed class of recombination-deficient mutants.Materials and Methods.-Solutions and media: TKCaB contains 10 gm Bacto-Tryptone (Difco), 5 gm KCl, and 0.015 M CaC12 in 1 liter of distilled H20. When growing thyminerequiring cells, 10 Ag/ml thymidine is added. Tryptone broth contains, per liter: 10 gm tryptone, 5 gm NaCl, 0.01 M MgSO4, and 20 mg thymidine. m3XD contains 3 gm NH4Cl, 0.3 gm MgS04 7H20, 15 gm Difco Casamino acids, 24 ml glycerol, 1 mg gelatin, 0.9 gm KH2P04, 2.1 gm