Genes involved in the determination and differentiation of segmental identity in Drosophila melanogaster have been identified by genetic interactions with the homeotic genes already known to be required. Many new genes required for transcriptional activation or repression of the homeotic genes have been identified. We have cloned two of the genes required for transcriptional activation and characterized their developmental functions in more detail. One of the genes that we identified as a transcriptional activator of the homeotic genes, the kismet gene, encodes a family of large nuclear proteins. Maternally-encoded kismet proteins are ubiquitous at the blastoderm stage and are required for proper segmentation of the embryo. After gastrulation, zygotically-encoded kismet proteins accumulate to the highest levels in the brain and central nervous system. The kismet proteins all have a common carboxyl-terminal domain of about 200 kilodaltons. Within this common region is a 41 amino acid domain, the BRK domain, which is also present in the brahma protein, a subunit of a chromatin-remodeling complex also required for transcriptional activation of homeotic genes. Diversity among kismet proteins is generated by the use of alternative 5 exons encoding different amino-terminal extensions. Two major classes of kismet proteins have been identified during embryogenesis. The smaller class proteins are about 230 kilodaltons in size, while the larger class are greater than 500 kilodaltons in size. In addition to the BRK domain conserved between kismet and brahma, the larger kismet proteins have another domain conserved in the brahma protein, a DNA-stimulated ATPase domain found in several subunits of chromatin-remodeling factors. The most closely-related proteins to kismet are the CHD family proteins, which have two chromodomains (also found in the larger kismet proteins) in addition to the DNA- stimulated ATPase domain. Last year we showed that another CHD family protein in Drosophila, dMi-2, is involved in silencing homeotic genes. We have also continued our characterization of the osa gene, which is also required for transcriptional activation of homeotic genes in Drosophila. Osa encodes a large nuclear protein with homology to the yeast SWI1 protein. The yeast SWI1 protein is part of a chromatin-remodeling complex that includes homologues of two other proteins that we have previously studied, the Drosophila brahma and moira proteins. We have shown that one of the alternative promoters of the Antennapedia homeotic gene requires high levels of osa, brahma, and moira proteins, and that mutations in these three genes show strong genetic interactions. In addition, we have isolated mutations in several other genes that interact with these proteins in regulating transcription from this Antennapedia promoter. We are currently cloning two of the new genes identified in these genetic screens. - Drosophila, homeotic, transcription, brahma, kismet, osa, moira
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