Discovery of Homeodomain

If we stroll back in the memory lane , History was created in 1983 with the discovery of Homeodomain independently by two groups headed by Walter Gehring and his colleagues at the University of Basel, Switzerland, and Matthew Scott and Amy Weiner, who were then working with Thomas Kaufman at Indiana University in Bloomington. Homeodomain is 180bp long, codes for a conserved motif which binds to DNA in sequence specific manner.The Homeodomain contains three well defined alpha helices and a more flexible fourth helix.Helices 2 and 3 form a helix-turn-helix motif virtually identical to one found in prokaryotes repressors.

The recognition helix(3) interacts directly with major groove of DNA but additional interactions are also observed between loop immediately preceding the helix turn helix motif and DNA backbone.The flexible amino terminus of Homeodomain establishes contact with minor groove of DNA.Homeobox genes encode for transcription factors ,which regulates gene expression usually in association with other proteins in the complex at the promoter region of the gene.Homeodomain was first discovered in Hox genes but later were associated to other genes as well.So all Homeodmain containing genes are not homeotic genes but all homeotic genes posses homeodomain.

Discovery of Homeodomain – A historical account
Homeotic genes played a pivotal role in understanding the genetic control of development. Regulator genes were first discovered by pioneering work of Jacob and Monad in 1961,which are very important for activity of target genes.In Eukaryotes there are no Operons and finding regulator genes were difficult.In Drosophila a class of mutations exists called Homeotic mutations which are likely to represent regulator genes involved in genetic control of development.The first Homeotic mutant Bithorax was observed by Bridges in 1915 and later shown by EB LEWIS in 1978 to be a part of complex of genes called Bithorax complex. Homeotic mutants transforms one part of body into other suggesting they might be involved in genetic control of development.

Walter Gehring discovered a spontaneous homeotic mutant and named it Nasobemia.In this mutant antenna was transformed into leg,later the mutant was physically mapped to Antennapedia(Antp) gene.Cloning of antennapedia was hampered by technical difficulties but things improved with the time.

Chromosomal walking (Method developed by group of David Hogness) helped Gehring to clone Antp with the help of his post docs Richard garber and Atushi Kuroiwa.They were able to clone antp gene along with few other genes near to it after nearly four years of hard work.The first sign of Homeodomain was observed when chromosomal DNA from walk outside the region of Antp gene was cross hybridized with the cDNA complimentary to chromosomal region of Antp gene.The cross hybridizing gene was fushi tarazu , a segmentation gene next to antp.

William McGinnis then joined the lab of Gehring and searched for whole genome of Drosophila for cross hybridizing DNA sequences with various segments of antp and ftz and he found many genes binding to this sequence.Finally they tested the 3′ end of sequence of antp and ftz which was hybridizing with many genes with two clones of ultrabithorax,another Hox gene to see whether other hox genes have this sequence. McGGinis , Ernst Hafen and Michael Levine showed that last exon of ultrabithorax also cross hybridizes with those sequences of antp and ftz.They knew that they have discovered something very important and named it Homeobox. But they might not have imagined what they have discovered would be something which will connect all the organisms on earth.Now we know that homeodomain proteins exists in all living organisms even in fungi and plants and Hox proteins (Homologous to drosophila Hox) are reported in Cnidarians also,further strengthening the fact that all organisms are connected in some way.

The discovery of Homeodomain is considered to be first major discovery of evo devo because with the help of Homeodomain only researchers could identify that higher vertebrates like humans and other animals share the same counterparts which make the body parts of Drosophila.This was a huge discovery in those times when people refused to believe invertebrates like fly can have same starting material for development like human beings.

“Until then,” ,”there had been strong sentiment that animals as different as vertebrates and invertebrates would have such different mechanisms of control and growth and patterning that, at most, we could use analogies to compare them, but little else would be directly transferable.”

With the knowledge that the same mechanisms were at work, “the whole problem of development was simplified,” “Instead of having to study genes and proteins and molecular events that are very specific to one tissue, or one developmental stage, or one organism, you can take advantage of the special attributes of whatever your experimental system is, and whatever you learn is likely to be very useful in understanding a great many other events in other tissues, stages, and creatures.” —MP SCOTT

Sean Carroll views about similarity between Fruit fly and vertebrates body plan : “Looks are really deceiving ,looks were leading us in a totally wrong direction………the anatomy appeared to be different but there are similar/ fundamental genetic instructions underlying the building the whole animal kingdom”.

Source for Historical account: Guide book to Homeobox genes. Edited by Denis Duboule

Original articles:

1) McGinnis W, Levine MS, Hafen E, Kuroiwa A, Gehring WJ. A conserved DNA sequence in homoeotic genes of the Drosophila Antennapedia and bithorax complexes. Nature. 308(5958):428-433. [PubMed]

2) McGinnis W, Hart CP, Gehring WJ, Ruddle FH. Molecular cloning and chromosome mapping of a mouse DNA sequence homologous to homeotic genes of Drosophila. Cell. 1984 Oct;38(3):675-680. [PubMed]

3) McGinnis W, Garber RL, Wirz J, Kuroiwa A, Gehring WJ. A homologous protein-coding sequence in Drosophila homeotic genes and its conservation in other metazoans. Cell. 1984 Jun;37(2):403-408. [PubMed]