The expression of a Balbiani ring 1 gene that codes for a salivary gland-specific 180-kD secretory polypeptide (sp180) is regulated developmentally. Immunoblots of salivary gland protein incubated with an affinity-purified nonapeptide-reactive antibody demonstrated that the salivary gland content of sp180 increases as much as 10-fold between stages 8 and 10 of the fourth larval instar. Hybridization of RNA dot-blots with an oligonucleotide probe indicated that the observed increase in sp180 was preceded by a parallel 20-fold increase in the steady state level of its mRNA beginning between stages 7 and 8. In vitro nuclear transcription experiments demonstrated that there was a 10-fold acceleration in the rate of sp180 gene transcription between stages 6 and 10. The limited period of expression of the sp180 gene contrasted dramatically with the expression of Balbiani ring genes BR1, BR2 alpha, BR2 beta, and BR6, which code for the sp-I family of fibrous secretory polypeptides. The appearance of sp180 in secretion coincided with microscopically visible changes in the bundling of these fibrous polypeptides. At the same time, we noticed changes in the appearance and consistency of feeding tubes that larvae construct with this secretion. These results lead us to propose that sp180 may modify the structure or utilization of fibrous secretory polypeptides specifically for the assembly of pupation tubes necessary for larval/pupal ecdysis.

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