The 68C intermolt puff of Drosophila melanogaster contains a cluster ofthree glue protein genes, Sgs-3, Sgs-7, and Sgs-8. Analyses of chromosomalrearrangments which break near the glue gene cluster show that a region of nomore than 20 kilobase-pairs (kb) is required for expression of the genes and forformation of the 68C puff. This result is supported by P-element-mediated-transformationexperiments in which defined segments of the 68C region areintroduced back into the fly genome. Based on the criteria of correct tissue- andstage-specific expression, transcription of an RNA of appropriate size andabundance, and production of an sgs-3 protein, the correctly regulated expressionof the Sgs-3 gene requires less than 3.4 kb of total flanking sequences,approximately 2.3 kb 5' and 1.1 kb 3'. When upstream sequences are truncated at130 base-pairs, low levels of Sgs-3 expression are observed in some cases, withnormal tissue- and stage-specific expression retained. Formation of a newintermolt puff at the site of insertion is observed for transformants in which theintroduced DNA contains all three 68C glue genes, but not for those which containonly an introduced Sgs-3 gene, even for cases in which the gene is abundantlyexpressed.
An attempt was made to recover lethal mutations in genes closely flankingthe 68C glue protein genes by screening mutagenized chromosomes over largedeficiencies which delete the 68AC region. Although a large number of lethal andsemi-lethal mutations were recovered, including many which define newcomplementation groups, none maps close to the region of the 68C glue genecluster.
