Cloning of the homeotic Sex combs reduced gene in Drosophila and in situ localization of its transcripts.

We have extended our ;chromosomal walk' in the Antennapedia-complex (ANT-C) by isolating overlapping DNA sequences spanning the chromosomal segment between Antennapedia (Antp) and Deformed (Dfd). The transcription units, homeoboxes and M-repeats were mapped within this region. Four transcription units Antp, fushi tarazu (ftz), Sex combs reduced (Scr) and Dfd contain both a homeobox and an M repeat, whereas at least two additional transcription units, x and z, were found to lack these elements. The Scr locus was identified by deletion mapping. It consists of at least two exonic regions separated by a large intron. The homeobox is located in the 3' exon and is 82% homologous to the one in Antp. Scr encodes a major 3.9-kb RNA. A corresponding cDNA clone was used as a probe for in situ hybridization to sections of various embryonic stages. At gastrula stages Scr transcripts accumulate in the posterior head and the anterior thoracic region of the germ band. At later stages a strong accumulation of transcripts is observed in the suboesophageal and the prothoracic ganglion of the ventral nervous system.