FORKED PROTEINS ARE COMPONENTS OF FIBER-BUNDLES PRESENT IN DEVELOPINGBRISTLES OF DROSOPHILA-MELANOGASTER

The forked (f) gene of Drosophila melanogaster encodes six different t ranscripts 6.4, 5.6, 5.4, 2.5, 1.9, and 1.1 kb long. These transcripts arise by the use of alternative promoters. A polyclonal antibody rais ed against a domain common to all of the forked-encoded products has b een used to identify forked proteins on two-dimensional sodium dodecyl sulfate-polyacrylamide gel electrophoresis gels and in Drosophila pup al tissues. The antibody stains fiber bundles present in bristle cells for about 15 hr during normal pupal development. Electron microscopy shows that these fibers are present from 40 to 53 hr in bristles of wi ld-type flies but are absent in the nullf36a mutant. The forked protei n(s) thus appear to be an essential part of the bristle fibers. The ph enotype of the f36a mutation can be rescued by a 13-kb fragment of the forked locus containing the coding regions for the 2.5, 1.9, and 1.1- kb transcripts, suggesting that the proteins encoded by the three larg e forked RNAs are dispensable during bristle development. Increasing t he copy number of a P[w+,f+] construct containing the 13-kb fragment i nduces a hypermorphic bristle phenotype whose severity correlates with the number of copies of P[w+,f+] present. These results indicate that alterations in the ratios among the forked proteins, or between forke d products and other components of the fiber, result in abnormal assem bly of the fibrillar cytoplasmic structures necessary for bristle morp hogenesis.