Isolation and characterization of a new family of mobile dispersed genetic elements, mdg3, in Drosophila melanogaster

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Abstract
The properties of a new family of mobile dispersed genetic elements, mdg3, are described. Mdg3 is represented by a DNA fragment of about 5.5 kb long which is framed with two repetitive sequences 300–500 base pairs in length. Virtually the whole region of mdg3 including the repetitive sequences is transcribed and, as the result, 26S poly(A)+RNA is formed. The repetitive sequence is present at least at the 3′-end of a major 26S transcript. Both strands of mdg3 are transcribed, but one direction is predominant. Besides 26S RNA, a minor transcript, 15S poly(A)+RNA, can be observed. 15S RNA does not contain sequences corresponding to the middle of mdg3. Possibly it is formed from 26S RNA by means of splicing. About 15 copies of mdg3 are present in the genome of embryonic cells and polytene chromosomes. Different copies of mdg3 are identical but surrounded with different DNA sequences. In culture cells, the gene is amplified and the number of copies is increased ∼ 13 times. Mdg3 is localized in 15–17 sites on different chromosomes of D. melanogaster. Its location varies from one animal to another. The properties and possible nature of mobile dispersed genetic elements are discussed.