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Six major transcripts encoding the α, β, and γ subunits of the larval serum protein LSP-1, the single subunit of the LSP-2, and the P1 and P6 polypeptides appear sequentially in the fat body tissue during the third larval stage of Drosophila melanogaster. Northern blot analysis shows that the transcription of the LSP-2 and P1 mRNAs is restricted temporally to the third instar and the early phase of pupal development and spatially to the fat body tissue. The synthesis and/or accumulation of the LSP-2 and P1 transcripts are differentially affected by mutations of the dor and occ loci of the 2B5 complex of genes involved in the trans-regulation of the activity of ecdysteroid-responsive genes. Application of the P element-mediated germ line transformation to the P1 gene shows that correct developmental expression of this gene requires no more than 1.5 kb of DNA sequences upstream from the mRNA start site.