Sequence Arrangement in Satellite DNA from the Muskmelon

Abstract

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.degree. 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 EM at this temperature. This observation provides direct evidence that the 2 fractions are not closely linked. Apparently satellite I, the high tm, [temperature at which half the DNA has denatured] 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 the assay, apparently the minimum size of each unit of satellite II is 2.5 .times. 107 daltons. It is unlikely that any spacer sequences are interspersed with either satellite. Sequences homologous to those of satellite I were 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 2 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. The results are interpreted as an observation of a recently created family of tandemly repeating sequences whose members are beginning to be scattered throughout the genome. 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.