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— — — CC/NUMBER 31 This Week’s Citation Classic AUGUST 1,1983 [irown I) D & Dawld I B. Specific amplification in oocytes. I Science 160:272-80, 1968. IDepartment of Embryology, Carnegie Institution of Washington, Baltimore, MD) The for 18S and 28S ribosomal RNA are “An international meeting on the nucleo- amplified specifically in oocyte nuclei of amphib- lus was held in Montevideo, Uruguay, in iarss forming more than a thousand nucleoli in each nucleus. These extra genes support enormous 1965. Without a doubt, the highlight of that rates of ribosomal RNA synthesis during meeting was Birnstiel’s demonstration of oogenesis. [The SCI® indicates that this paper has how he had used physicochemical tech4- been cited in over 530 publications since 1968.1 niques to isolate the ribosomal RNA genes. At that conference I heard Oscar Miller, then a staff member at the Oak Ridge Labo- Donald D. Brown ratories, describe the presence of circular chromosomes in the many nucleoli of frog Department of Embryology 5 Carnegie Institution of Washington oocyte nuclei. I knew instantly from the Baltimore, MD 21210 previous correlations of ribosomal RNA genes and the that these must be July 7, 1983 extra copies of ribosomal RNA genes. lgor Dawid, a fellow staff member at Carnegie, “This paper and one published indepen1 - and I set out to prove this idea. dently at the same time by Joseph Gall “A key experiment described in our Sci- were the first to demonstrate specific gene ence paper depended upon the isolation by amplification — an event programmed into hand of ten thousand nuclei from Xenopus the development of a cell. The genes are Iaevis oocytes, a technique perfected by those for ribosomal RNA in oocytes of the Mrs. Eddie Jordan, my colleague at Carne- amphibian Xenopus Jaevis. gie. We used buoyant density and hybridiza- “My own involvement in this kind of re- tion methods to identify the amplified genes search dates from 1960 when I first began to in extracts from these nuclei. Oocytes of study gene expression in frog embryos at the amphibians, fish, and certain insects are Carnegie Institution’s department of em- known to amplify their ribosomal RNA bryology in Baltimore. It had just become genes, and there are now examples of ampli- possible to measure RNAs as direct gene fication of genes for proteins. A phenome- products, and the first RNAs to be purified non that I termed ‘forced gene amplifica- were the stable ribosomal and transfer tion’ is a response by which cells become RNA5 that comprise the bulk of cellular resistant to a drug by amplifying the gene RNA. In 1964, John Gurcion and I found that whose product is interfered with by the a mutant of Xenopus that affects the num- drug. It is apparent that this is an important ber of nucleoli was defective in the synthe2 - cause for resistance to chemotherapy. Our sis of the two large ribosomal RNAs. This own search for other genes that might be confirmed that the nucleolus is the site of amplified during development was negative synthesis of ribosomal RNAs, a correlation causing us to focus on other kinds of gene that had already been made cytologically. control during development. Max Birnstiel, then in Edinburgh, demon- “The reference has been cited because it strated that this mutation was a deletion of represents the first observation of one im- the several hundred genes3 that encode these portant mechanism for gene regulation in two ribosomal RNA5. This crucial paper set animal cells. Since that time, there have the stage for the isolation of these genes, been numerous other examples of gene am- the very first instance of gene isolation from plification in other animal cells. (For a any living organism. review, see reference 6.)”

I. GaUl G. Differential synthesis of the genes for ribosomal RNA during amphibian oigenesis, Proc. Na:. Acad. Sci. IfS 60:553-60. 1968. IThe Sc! indicates that this paper has been cited in over 280 publications since 1968.1 2. Brown D 0 & Gurdon I B. Absence of nbosomsl RNA synthesis in the anucleolate mutant of Xenop~slaevis. Proc. Nat. Acad. Sci. US 51:139-46, 1964. 3. Wallace H & Birnatlel M L. Ribosomal cistrons and the nucleolar organizer. Biochim. Biophys. Acts 114:296-310, 1966. 4. Bhrusllel M L, Wallace H, SbIln I L & Fbcbberg M. Localization of the ribosonjal DNA complements in the nscleolar organizer region of Xenopus ben:. Na,. CancerIns,. Monogr. 23:431-47, 1966. 5. MIller 0 I, Jr. Structure and composition of peripheral nucleoli of salamander oocytes. Nat. Inst. Monogr. 23:53-66, 1966. 6. Brown D D. Gene expression in eukaryotet. Science 211:667-74. 1981.

CURRENT CONTENTS® LS 23 ® 1983 by SI®