Ultrabithorax

Ultrabithorax or Ubx is a member of the homeobox gene family. Homeobox proteins function as transcription factors. In Drosophila melanogaster it is expressed in the third thoracic (T3) and first abdominal (A1) segments and represses wing formation. The Ubx gene regulates the decisions regarding the number of wings and legs the adult Drosophila melanogaster will have. Ubx is activated when there is a certain lack of Hunchback protein. The concentrations of Hunchback only exist in the anterior and posterior regions of the embryo, therefore Ubx is expressed only in middle segments. Thus, the hb gene may play an important role in the specification of the boundaries of Ubx expression.

The Ubx gene contains a 5' exon, two micro-exons, an optional B element, and a C terminal exon. The Ubx genomic DNA length is 76 kb and its cDNA clone length is 3.2 to 4.6 kb. The 5' exon contains the 5'UTR which has 964 bases. The C terminal exon contains the 3'UTR which has 1580 to 2212 bases.

The developmental role of Ubx is determined by the splicing that takes place during development. Certain splice factors of a particular cell allow that cell to regulate the developmental fate of that cell by making different splice variants of transcription factors. In D. melanogaster, at least six different isoforms of Ubx exist.

Mutations of the Ubx gene will lead to transformation of dorsal and ventral appendages of the third thoracic segment(T3), which includes the haltere and third leg, into the counterparts on the second thoracic segment(T2). If Ubx is present in T3, it will prevent the original fate of the T2 segment. Such mutations can produce a second set of wings.

Target genes
Ubx targets hundreds of different genes at different stages of morphogenesis including regulatory genes such as transcription factors, signalling components and terminal differentiation genes.

Ubx represses selected Dpp target genes in the anterior and posterior axis. Ubx represses Wingless in the posterior compartment of the dorsoventral axis. Ubx also selectively represses one enhancer of the vestigial genes in the proximodistal axis.

Regulation
Expression of Ubx is repressed by the long non-coding RNA Bxd.

Ubx self-assembly
Interestingly, Ubx can also form materials in vitro besides its well known function as a transcription factor. Macroscale materials in the form of fibers, ropes and sheets can be generated from recombinant Ubx protein which self-assembles under gentle conditions. Ubx materials are mechanically robust. By altering fiber diameter, the breaking strength, breaking strain, and Young’s modulus can be tuned to values spanning an order of magnitude, ultimately changing the mechanism of extension.

Ubx nanocomposites
Incorporation of nanoparticles during the hierarchical self-assembly of Ubx-based materials can impart function to the resulting composite materials. Majithia ''et. al.'' have shown that CdSe-ZnS core-shell quantum dots can be incorporated into macroscale Ubx films and fibers thus imparting optical properties in the resulting nanocomposites. Quantum dots can incorporated in Ubx fibers at different time points during self-assembly. When introduced prior to self-assembly, qunatum dots favorably interact with Ubx monomers yielding composite fibers with a rough surface morphology. Conversely, quantum dots templated onto the Ubx film post-self-assembly can be subsequently drawn into smooth composite fibers. Additionally, the quantum surface charge impacts its distribution within the composite material. Negatively charged quantum dots favorably interact with the positively charged Ubx backbone leading to a homogenous distribution in Ubx∙QD composite fibers. Quantum dots with either positively or negatively charged coatings significantly enhance Ubx∙QD fiber extensibility.