Differential expression during embryogenesis of three genes clustered in the Ras1 region of Drosophila melanogaster.

Transcription mapping and nucleotide sequence analysis reveal that the genomic region of the Drosophila Ras1 gene contains a cluster of three closely localized genes. A gene termed Rlb1 is located nearby and upstream of Ras1, and is oriented in the opposite polarity relative to Ras1. In addition, a third gene termed Rlc1, is found at a very close proximity downstream to Rlb1. Ras1, the homologue of the human transforming ras genes, has been shown to be active in the posterior termini of the Drosophila embryo and in the eye imaginal disc in pathways of cell fate determination. We demonstrate that during embryogenesis Ras1 transcripts are restricted mainly to the embryonic central nervous system, suggesting that the gene product also may have a role in these nerve cells. Rlb1 encodes for a novel, lysine-rich basic protein. It is expressed mainly in the developing midgut and in the somatic mesoderm. Rlc1 also encodes for a novel, basic protein. The expression of Rlc1 during embryogenesis is similar, but not identical, to the expression pattern detected for Ras1. The vertebrate p21Ras proteins are bound to the inner face of the cell membrane. Ras1, the Drosophila homologue of p21, and the Rlb1 protein, are also non-cytoplasmic, membranous proteins. Rlb1 is found in the cell membrane of larval midgut epithelial cells. In addition, Rlb1 is detected in the nuclei of these cells, and in the nuclei of the midgut imaginal cells.