Two new homeobox genes, PBX2 and PBX3, were isolated on the basis of their extensive homology to PBX], a novel human homeobox gene involved in t(l;19) translocation in acute pre-B-cell leukemias. The A sequence of 180 bp termed the homeobox was initially discovered as a conserved segment of the Antennapedia, Ultrabithorax, and fushi tarazu transcription units (40,41,53). Subsequently, homeoboxes have been observed in the genomes of many species by cross-hybridization and have also been found fortuitously in genes isolated by other means. Homeoboxes code for a conserved protein motif, termed the homeodomain, and the degree of similarity between different homeodomains has been used to group them into related classes (52). While most of the Drosophila homeodomain proteins have been implicated in developmental processes, the functions of the majority of non-Drosophila homeodomain proteins remain unknown. However, because of the presence of homeodomains in known transcription factors and the structural similarity between the homeodomain and DNA-binding motifs in bacterial proteins, the current view is that most if not all of these proteins function as sequence-specific DNA-binding proteins that likely play an important role in transcriptional regulation (for reviews, see references 17 and 52).Recently, a novel human homeobox gene, PBXJ, was identified because of its involvement in t(1;19) chromosomal translocation in acute pre-B-cell leukemias (29,47). This translocation results in the formation of a fusion transcript that codes for E2A-Pbxl chimeric proteins, in which the C-terminal region of E2A, which contains its basic and helix-loop-helix (bHLH) DNA-binding and dimerization motifs, is replaced by sequences from Pbxl that contain a highly divergent homeodomain. The role of E2A-Pbx chimeric transcription factors in the pathogenesis of acute pre-B-cell leukemias remains unknown.Considering the high level of divergence between Pbxl and previously reported homeodomains, we hypothesized that there may be additional, as-yet-uncharacterized homeobox genes which encode homeodomains similar to that * Corresponding author.of Pbxl. We describe here the characterization of two such genes, designated PBX2 and PBX3, which were isolated as a result of their cross-hybridization with the PBX] homeobox. A striking feature of the Pbxl, Pbx2, and Pbx3 proteins is their extensive sequence identity both within and outside of their homeodomains. The similarities in their structures and patterns of expression suggest a generalized, overlapping function for Pbx homeodomain proteins in most cell types.
MATERIALS AND METHODSIsolation and sequencing of PBX2 and PBX3 cDNA clones. PBX2 and PBX3 cDNA clones were identified by screening a HeLa cDNA library under nonstringent conditions with a PBXJ homeobox probe (nucleotides 1951 to 2210) (47). Library filters containing phage DNA were prehybridized overnight at 37°C in 43% formamide, 5x SSC (lx SSC is 0.15 M NaCl with 0.015 M sodium citrate), 5x Denhardt's solution, 1 mM sodium pyrophosphate...