Degree Name

PhD (Doctor of Philosophy)


Biomedical Sciences

Date of Award

December 1993


The L2Hs sequences are a polymorphic, interspersed, middle repetitive DNA family unique to human genomes. Genomic fingerprinting indicates that these DNAs vary from one individual to another and between tissues of the same individual. Sequence analysis reveals that they are AT-rich (76%) and contain many unusual sequence arrangements (palindromes, inverted and direct repeats). These sequence properties confer on the L2Hs elements the potential to fold into non-B-form structures, a characteristic of recombination hot spots. To test this hypothesis carbodiimide, osmium tetroxide and S$\sb1$ nuclease were used as single-strand specific probes to study a recombinant plasmid, pN6.4.39, containing a single L2Hs segment. Different forms of the plasmid substrate were analyzed, including linear molecules and circular forms of low, intermediate and high superhelical densities. In addition, plasmid DNA in growing E. coli cells were analyzed. Modified plasmid DNA was analyzed by primer extension in a sequencing-type reaction format. These studies demonstrate that the L2Hs sequences: (1) assume non-B-form structures both in vitro and in vivo, (2) map to predicted cruciform structures, (3) behave as C-type extrusion sequences, and (4) that these unusual DNA structures are dependent on plasmid superhelicity.

Document Type

Dissertation - unrestricted