Developmental regulation of the Drosophila tropomyosin II gene in different muscles is controlled by muscle-type-specific intron enhancer elements and distal and proximal promoter control elements.

Transcriptional control of the Drosophila tropomyosin II gene muscle promoter has been investigated by expressing TmII promoter lacZ reporter gene constructs in P-element-mediated transformed flies. A TmII/lacZ reporter gene containing 243 bp of upstream sequence, the first exon, the first intron, and 72 bp of the second exon was expressed in all muscles of embryos, larvae, and adults. Deletion of upstream sequences between -243 and -22 bp only reduced the levels of transgene expression in muscle while deletion of the intron eliminated expression. Analysis of deletions within the first intron indicated that a 454-bp muscle enhancer region, from +167 to +621, was required for high levels of transgene expression in all larval and adult muscles. When this region was deleted low levels of expression still occurred in larval and adult somatic and visceral muscles; however, there was no detectable expression in adult indirect flight and jump muscles. The 454-bp muscle enhancer region was also able to drive muscle-specific expression when placed upstream of a heterologous hsp70 promoter; however, three subfragments of the 454-bp region were unable to drive expression of the hsp70 promoter, suggesting that this region may contain multiple interacting cis-acting elements. Interestingly, the 454-bp region was inactive when placed upstream of a TmII promoter construct containing upstream DNA and most of the first exon but was active when additional exon and intron DNA was included, indicating that additional promoter elements are located in this region. Thus TmII transcription is controlled by multiple muscle type-specific cis-acting control elements and upstream and downstream promoter control elements.