Evolution of Myb

Three closely related Myb genes are present in vertebrate animals including mammals, birds, amphibians, and bony fish.  In the laboratory mouse, c-Myb/MYB is required for blood cell production in the embryo and for intestinal maintenance in adults, A-Myb/MYBL1 is required for spermatogenesis and mammary gland proliferation, and B-Myb/MYBL2 is required very early in embryonic development.  The invertebrate sea urchin and fruit fly each have a single Myb gene most closely related to B-Myb/MYBL2 of vertebrates.

animal myb tree

The DNA-binding domains of these Myb proteins of animals all contain three tandem repeats of ~50 amino acids each (blue boxes) located near the amino terminus (left end).  In addition, all of these proteins contain a negative regulatory domain (green box) located near the carboxyl terminus (right end).  The A-Myb and c-Myb proteins contain an evolutionarily conserved central transcriptional activation domain (red box) that is not present in B-Myb or the Myb proteins of invertebrates.  The corresponding region of B-Myb has continued to evolve rapidly. These results suggest that A-Myb and c-Myb arose by two gene duplications (red circles) that occurred after the emergence of the vertebrate lineage.  In support of this hypothesis, we have found that B-Myb, but neither A-Myb nor c-Myb, can complement the hemocyte (blood cell) defects in Drosophila larvae that lack a functional Myb gene.  Surprisingly, the intensively studied nematode worm Caenorhabditis elegans lacks a closely related Myb gene.  This gene appears to have been lost during the the evolution of nematodes, because simpler invertebrate animals including the sea anemone do have a Myb gene that is closely related to the Myb genes of vertebrate animals.

animal myb proteins

All green plants contain one or a few different genes encoding Myb proteins with a DNA-binding domain composed of three tandem repeats.  In addition, flowering plant species each contain over a hundred different genes encoding Myb proteins with a DNA-binding domain containing only two tandem repeats.  Cellular slime molds and some species of fungi also contain Myb proteins with a DNA-binding domain composed of three tandem repeats.

eukaryotic myb proteins

More distantly related proteins containing one or more Myb repeats are present in all eukaryotes (organisms with nucleated cells).  This larger family of Myb-related proteins include components of machines that remodel nucleosomes (SWI3 and ISWI), that covalently modify histone tails (ADA2 and NCoR), that regulate RNA polymerase III (TFIIIB”), and that bind telomeric DNA sequences (TRF1, TRF2, TAZ1, and RAP1).  These more widely divergent Myb-related proteins are sometimes referred to collectively as “SANT domain proteins”.  Thus far, Myb-related genes have not been found in prokaryotes (bacterial organisms that lack a cell nucleus).