The organization and amplification of two chromosomal domains containing Drosophila chorion genes.

Drosophila chorion genes are located in two clusters, one on the X chromosome at 7F1-2 of the polytene chromosome map and a second on the third chromosome at 66D11-15. Genes in both regions undergo amplification in ovarian follicle cells prior to their expression late in oogenesis. Analysis of cloned genomic segments derived from these chromosomal sites revealed that each cluster contains two tandemly transcribed chorion protein genes separated by only 1-2 kb. At least two other regions complementary to ovary RNA are located within 5 kb of these genes. During oogenesis, the transcribed sequences within each cluster, as well as the spacer sequences that separate them, are amplified equally. Sequences adjacent to the transcribed regions also replicate differentially but to a lesser extent, giving rise to gradients of decreasing amplification involving 40-50 kb of flanking chromosomal sequences. Differences between the restriction maps of unamplified and amplified DNA could not be detected in genomic DNA within either of the 90-100 kb domains of amplification. These observations suggest a model of amplification in which additional rounds of replication are specifically initiated within the central gene-containing regions, followed by bidirectional replication in the absence of discrete termination sites.