Abstract

The DNA sequence motif ATTTGCAT (octamer) or its inverse complement has been identified as an evolutionarily conserved element in the promoter region of immunoglobulin genes. Two major DNA-binding proteins that bind in a sequence-specific manner to the octamer DNA sequence have been identified in mammalian species--a ubiquitously expressed protein (Oct-1) and a lymphoid-specific protein (Oct-2). During characterization of the promoter region of the chicken immunoglobulin light chain gene, we identified two homologous octamer-binding proteins in chicken B cells. When the cloning of the human gene for Oct-2 revealed it to be a member of a distinct family of homeobox genes, we sought to determine if the human Oct-2 cDNA could be used to identify homologous chicken homeobox genes. Using a human Oct-2 homeobox-specific DNA probe, we were able to identify 6-10 homeobox-containing genes in the chicken genome, demonstrating that the Oct-2-related subfamily of homeobox genes exists in avian species. Low-stringency screening of a chicken embryonic cDNA library allowed us to clone one of these genes. DNA sequence analysis revealed it to be the chicken homologue of the human Oct-1 gene. The predicted protein sequence of the chicken Oct-1 gene demonstrated that the gene for Oct-1 has been highly conserved during vertebrate evolution with an overall 96% amino acid sequence identity between the chicken and human proteins. The previously described POU domain (termed POU for its presence in the Pit-1, Oct-1/Oct-2, and Unc-86 genes) and homeobox domain are 100% conserved between the two protein products. Together, our data show that the POU-containing subfamily of homeobox genes have been highly conserved during vertebrate evolution, apparently as a result of selection for their DNA-binding and transcriptional regulatory properties.