REPEATING UNITS OF XENOPUS-LAEVIS OOCYTE-TYPE 5S DNA ARE HETEROGENEOUS IN LENGTH

The restriction endonuclease enzymes Hind III and Hae III cleave Xenopus laevis 5S DNA at 1 and 3 sites, respectively, in each repeating unit of approximately 700 base pairs. The cleavage sites for both enzymes were located within the repeating unit by denaturation mapping of the restriction fragments. The Hind III products and 1 of the Hae III fragments are variable in length, indicating heterogeneity in the length of the repeating unit in 5S DNA. This length heterogeneity is confined to the major adenine + thymine [A + T] rich spacer region. Repeating units differ from each other by discrete quanta of approximtely 15 base pairs. The A + T rich spacer consisted largely of tandem subrepeats of just this size. The repeat-length heterogeneity is due to variable numbers of these subrepeats in the spacer regions of the major repeating units.