Characterization of a minichromosome derived from the transposing element TE1 in Drosophila melanogaster

• The transposing element TEI contains the structural genes white and roughest from the Drosophilr X-chromosome. These genes are flanked by FB-elements, which are responsible for the mobility. At one occasion the TE, probably together with an adjacent segment in chromosome 2. has formed a minichromosome. This chromosome contains both the structural genes, the FB-elements,some centromeric and/or telomeric heterochromatin. It probably has a centromere as well, as it is transferred to the offspring at a high rate. From this minichromosome a smaller one has originated, probably through the loss of the region from chromosome 2 and some heterochromatin. This smaller minichromosome has been characterized in the following way: 1. Size determination hy pulsed field gel electrophoresis. -The chromosome turned out to be little more than one megabase. 2. y-irradiation of DNA from the minichromosome. ~ The aim of this experiment is to find out if the chromosome is circular or linear. A radiation dose which causes one break within a circle ought to accumulate DNA of the same size as the minichromosome. In this case no accumulation occurred and thus the chromosome is probably linear. 3. Cloning of sequences from the minichromosome. ~ A low melting agarose gel was run and a fragment was cut out from a region which contained DNA fragments of the same size as the minichromosome. The DNA was cut simultaneously by EcoR I and Pst I and ligated into the vector pBS containing T7 and T3 primers. The ligated DNA was amplified by the PCR method, which rendered several fragments of varying size. These fragments were ligated into the vector pCR1000TM. Positive clones are being analysed at present. With these clones we intend to construct a map of the minichromosome.

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