DIFFERENTIAL MUSCLE-TYPE EXPRESSION OF THE DROSOPHILA TROPONIN-T GENE- A 3-BASE PAIR MICROEXON IS INVOLVED IN VISCERAL AND HYPODERMIC MUSCLE SPECIFICATION

The complete genomic organization of the Drosophila troponin T (TnT) g ene shows many interesting features, including the presence of a micro exon of only 3 nucleotides conserved among Drosophilidae, It is the sm allest bona fide exon so far described, placing a new lower limit on t he nucleotide number required for correct splicing, Four muscle-type s pecific transcripts are generated by developmentally regulated alterna tive splicing, Exons 3, 4, and 5 are absent in the transcript present in jump and flight muscles. A total of 11 exons are present in the adu lt hypodermic muscles transcript, whereas the microexon is absent in t he larval hypodermic musculature, The two isoforms differ in a lysine residue, Post-translational regulation of the flight muscles/tergal de pressor of the trochanter-specific isoform is involved in flight and/o r jump function. The interaction domains of TnT in the tropomyosin-tro ponin complex are strongly conserved in the known vertebrate and inver tebrate TnT sequences, whereas the terminal regions show an important variability, The COOH-terminal region shows important phylogenetic var iations, whereas the NH2-terminal domain is associated with specific m uscle types in a particular organism, a finding that discloses a selec tive value for these domains in the functionality of distinct muscles in different organisms.