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The Shaker locus of Drosophila contains a very large transcription unit. It is expressed predominantly in the nervous system by multiple, differential as well as alternative, splicing mechanisms into different, but functionally related proteins. The structure of the Shaker transcription unit and the properties of the encoded Shaker protein family provide a molecular basis for A channel diversity in excitable cells.

We have cloned 215‐kb DNA containing the maternal effect region (ME) of the Shaker gene complex (shC) at 16F of the Drosophila X chromosome. Five translocation and deletion breakpoints have been mapped on the cloned DNA allowing a correlation of the genetic map to transcription units. The ME region spans ˜100 kb. The genetic behavior of this region correlates with the occurrence of maternal RNAs in this part of the ShC. Two transcripts have been identified in the vicinity of chromosomal rearrangements which cause a Sh phenotype. These are a 4.5‐kb transcript interrupted by T(x;2)B27 and a 2‐kb transcript interrupted by T(X;3)ShLC and T(X;Y)W32. The latter transcript is derived from a primary transcript which spans >65 kb genomic DNA. The cDNA‐sequencing data show that this Shaker (IAchannel) gene can encode a protein of ˜35 kd with three α‐helical membrane‐spanning sequences near its carboxyl terminus. These have a striking homology with membrane‐spanning sequences of the vertabrate Na+ channel.

We have analyzed one of the nine complementation groups that constitute the haplolethal (HL) region of the Shaker gene complex {ShC). Five mutations, including a dominant lethal, define this complementation group: HL L Mutant phenotypes show abnormal embryogenesis with structural defects in the nervous system and aberrant degeneration of specific adult muscles in addition to altered action potentials. HL I encodes a family of proteins with extensive homology to invertebrate troponin I (Tnl). Members of this family are brought about by two alternative and two mutually exclusive exons in conjunction with two differential polyadenylation sites. Transcription analysis indicates that some isoforms are adult specific and others are synthesized throughout development, except during early metamorphosis. Certain isoforms of Drosophila Tnl are expressed in specific muscles. The specificity of mutant phenotypes suggests a functional role of particular Tnl isoforms in the development and the mature activity of muscle and nervous systems.

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