A Xenopus laevis gene encodes both homeobox‐containing and homeobox‐less transcripts.

A cDNA clone (p52) that contains all the protein‐coding region from the maternally expressed XlHbox 2 locus of the frog Xenopus laevis has been isolated and sequenced. A probe containing the exon preceding the homeobox detected transcripts which arise from a splicing event in which the homeobox‐containing exon is replaced by another exon lying 5′ to it in the genome. Both the homeobox‐containing and homeobox‐less splicing event occur in the same tissues, with the homeobox‐less RNA representing the minority of mRNA from this gene. There may therefore be a function for two types of transcript, and hence protein, from this locus. This phenomenon may not be exclusive to the XlHbox 2 gene of Xenopus, but might occur more generally in other homeobox‐containing genes. The protein deduced from the homeobox‐containing cDNA is significantly similar to the yeast mating type factor a1 (MAT‐a1) gene product. In addition to the previously described homology of the homeodomains, the amino‐terminal domains of XlHbox 2 and MAT‐a1 are similar to each other; thus essentially all of the MAT‐a1 protein corresponds to some part of the XlHbox 2 protein. In the case of XlHbox 2, the protein coded for by the homeoboxless mRNA would contain all of the non‐homeobox homology to yeast MAT‐a1.