Characterization, molecular cloning and sequencing of YP3 s1, a fertile yolk protein 3 mutant in Drosophila

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The three yolk proteins (YP1, YP2 and YP3) of Drosophila melanogaster are synthesized in two tissues of the adult female, the fat body and ovarian follicle cells. The YPs are selectively accumulated in the oocyte to provide nutrients for embryogenesis. We describe a female-sterile mutant, fs(1) A1526, which lacks YP3 in the haemolymph. The female sterility mutation mapped some distance away from the yp3 gene on the X chromosome and we were able to separate the YP3 defect from the female sterility by recombination, thus producing a fertile line of flies having no YP3 in the eggs. This shows that YP3 is not essential for embryogenesis. The mutant line is to be known as YP3s1. Investigation of yp3 transcription in the mutant females revealed that the gene is transcribed but yp3s1 mRNA levels are reduced relative to wild type. Transcription of the mutant yp3 gene can be induced in males by ecdysone. Investigation of the yolk proteins in YP3s1 females suggested that the YP3s1 polypeptide is synthesized in the fat body but not secreted. The mutant YP3 protein shows an increase in apparent molecular weight of approximately 1 kDa. The mutant yp3 gene was cloned and the DNA sequence determined. The sequence differences between the mutant and wild-type genes include an amino acid substitution in the leader sequence. We suggest that this may be responsible for the failure of YP3 secretion in the mutant YP3s1, and speculate on the cause of the reduction seen in the steady-state level of yp3 mRNA.