Ancestry and diversity of BEL1-like homeobox genes revealed by gymnosperm ( Gnetum gnemon) homologs.

BEL1-like homeobox genes encode plant-specific transcription factors, at least some of which are important for ovule development. Here we report MELBEL1-MELBEL4,the first BEL1-like genes from a non-flowering plant, the gymnosperm Gnetum gnemon. Our analyses suggest that there was already at least one BEL1-like gene present in the most recent common ancestor of extant seed plants about 300 million years ago. Multiple sequence alignments revealed that since this time, not only the DNA-binding homeodomain, but also a protein-protein interaction domain upstream of the homeodomain, termed the BEL domain, has been highly conserved. Sequence comparison of the BEL domain with upstream domains that have been conserved in other TALE homeodomain proteins, i.e. MEIS, KNOX, and PBC, revealed only weak sequence similarity. However, since homology has been shown for MEIS, KNOX, and PBC domains and since KNOX and BEL domains directly interact in vivo, it appears likely that the BEL domain was also derived from an ancestral upstream (MEINOX) domain.