Molecular analysis of the lethal(1)B214 region at the base of the X chromosome of Drosophila melanogaster.

Approximately 50 kb of genomic DNA was isolated from polytene chromosome bands 19F1 and 2 of Drosophila melanogaster. Bands 19F1 and 2 are in the immediate vicinity of the beta-heterochromatin at the base of the X chromosome and encompass the little fly-like and lethal(1)B214 complementation groups. The cloned DNA consists of an approximately 21 kb stretch of unique or low copy number sequence that is bounded by repetitive elements interspersed with further unique sequences. The presence of repeated sequences is characteristic of regions within and adjacent to beta-heterochromatin. At least part of a tRNA gene cluster is present within the 50 kb of cloned DNA. The cloned region also produces at least 18 discrete size classes of developmentally regulated poly(A)+ RNA species. A 2 kb EcoRI fragment (E10), which lies in the 21 kb stretch of unique sequence, generates seven of these transcripts (of sizes 3.5, 3.35, 2.1, 2.0, 1.5, 1.2 and 1.0 kb) in wild-type flies. However, a small deletion of approximately 75 bp in E10 in a lethal(1)B214 mutant allele is associated with alterations in the production or processing of all seven of these transcripts. These data identify E10 sequences as belonging to the lethal(1)B214 gene and suggest that the wild-type lethal(1)B214 gene encodes multiple transcripts. Furthermore, no transcripts of the same size and having the same developmental profile as those generated by the wild-type E10 fragment were identified by probes covering the remainder of the cloned region. This suggests that at least the larger transcripts hybridizing to E10 are partly transcribed from sequences located outside the cloned region, which indicates that the lethal(1)B214 gene extends for more than 20 kb and contains other transcriptionally active sequences within it.