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Articles

Sequence Arrangement in Satellite DNA from the Muskmelon ¹

^a Departments of Botany and Genetics, University of Washington, Seattle, Washington 98195

Two fractions of a satellite DNA from the muskmelon (Cucumis melo L.) isolated as a unimodal peak from CsCl gradients, differ in melting properties and complexity as estimated by reassociation kinetics. At 49.8 C, all of the low melting fraction was denatured and all of the high melting fraction was native. There were almost no partially denatured molecules detected in the electron microscope at this temperature. This observation provides direct evidence that the two fractions are not closely linked. We conclude that satellite I, the high t_m, low complexity fraction, exists as a 600-nucleotide sequence in blocks of at least 67 tandem repeats. Since the complexity of the low melting fraction, satellite II, is greater than the size of the molecules in our assay, we can only say that the minimum size of each unit of satellite II is 2.5 x 10⁷ daltons. It is unlikely that any spacer sequences are interspersed with either satellite.

Sequences homologous to those of satellite I were also shown to be present as a minor fraction on 4900 nucleotide pair fragments with main band DNA density. These long main band fragments probably contain in addition at least two repeated sequence elements unrelated to satellite I since they aggregate (form large network structures) when reassociated. Coaggregation of sheared ³H-satellite I with long main band DNA could not be attributed to contamination of main band with long satellite DNA. We interpret the results as an observation of a recently created family of tandemly repeating sequences whose members are beginning to be scattered throughout the genome.

We discuss how the aggregation technique may be generally useful for assessing linkage between a minor and a major DNA fraction when both fractions may be present in the initial DNA preparation. Applications for the technique include the search for DNA sequences in the nucleus which are homologous with chloroplast DNA and for Agrobacterium tumefaciens DNA in the nuclei of plant cells transformed to the tumor phenotype by the bacterium.

¹ This work was supported by National Science Foundation Grant GB41179 and National Institutes of Health Grant 1 R01 GM22870-01. W.C.T. was a predoctoral trainee supported by NIH Training Grant GM00182.