The positions of nucleotide sequences involved in the stage-and/or tissue-specific expression of structural genes in higher organisms are largely unknown. One might predict, however, that closely linked genetic loci which are not coordinately controlled will contain regulatory sequences in close proximity to the respective coding regions. During the third instar of larval development in Drosophila melanogaster, the hormone ecdysterone augments the expression of several genes in fat body tissue, one of which is the structural gene coding for larval serum protein-2, LSP-21-5. Although LSP-2 transcripts are detected only in third-instar larval fat bodies, a genomic clone containing the LSP-2 gene (clone 104) hybridizes to poly(A)+ RNA prepared from other tissues. We have now analysed clone 104 for other coding regions. Three transcripts, designated T1, T2 and T3, hybridize within the 12.5 kilobases (kb) of DNA flanking the 5′ end of the LSP-2 coding region. The T1 coding region is separated from the 5′ end of the LSP-2 coding region by no more than 2.5-3.5 kb. In contrast to the dramatic increase in the level of LSP-2 transcript induced by ecdysterone in third-instar larval fat bodies, the T1 transcript remains below the detectable level, indicating that expression of the two coding regions is not coordinately regulated.
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