Actin gene expression is modulated by ecdysterone in a Drosophila cell line

The steroid hormone ecdysterone induced characteristic and specific changes of morphology, enzymatic activities and protein synthesis in a Kc 0% Drosophila melanogaster cell line. To study the ecdysterone action at a molecular level, a Drosophila genomic library was screened by differential hybridization to poly(A)+ RNA from control and ecdysterone-treated cells. Two recombinant phages were selected for hybridizing very intensively with poly(A)+ RNA of ecdysterone-treated cells and very weakly with poly(A)+ RNA of untreated ones. These two clones (lambda Dm 1632 and lambda Dm A5A1) mapped at the 5 C locus on polytene chromosomes; they overlap for a 9000 base-pair sequence that contains an abundantly transcribed region in ecdysterone-treated cells of about 2000 base-pairs. This region permits the selection of mRNA that gives, after translation in vitro, two polypeptides identified as cytoplasmic actin II and III. We demonstrated that these two recombinant phages, hybridizing preferentially with poly(A)+ RNA of ecdysterone-treated cells, contain the 5 C actin gene. Poly(A)+ RNA prepared from various times of treatment of cells were electrophoresed on agarose gels, transferred to nitrocellulose paper and then hybridized with the cloned actin probe. Results of these experiments indicate that there is a sharp increase in the level of RNA coding for actin after ecdysterone treatment of the cell, and that there are two forms of actin-specific RNA in the D. melanogaster cells. Using genomic blots with specific probes derived from lambda Dm 1632, we show that there are six actin genes per haploid Drosophila cell genome contained on six EcoRI fragments, as in Drosophila embryos, indicating that there is no rearrangement of these sequences in cultured cells. Our results suggest that the expression of actin genes in D. melanogaster Kc 0% cells is modulated by ecdysterone.