Molecular organization of the maternal effect region of the Shaker complex of Drosophila: characterization of an I(A) channel transcript with homology to vertebrate Na channel.

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)Sh 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 (I(A)channel) gene can encode a protein of 35 kd with three alpha-helical membrane-spanning sequences near its carboxyl terminus. These have a striking homology with membrane-spanning sequences of the vertabrate Na channel.