Ultrabithorax or Ubx is a gene found in insects, a member of the homeobox gene family, and as many homeobox genes do, functions as a transcription factor. 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 flies will have. Ubx is activated when there is a certain lack of Hunchback (hb) protein. Significant 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.
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.
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.
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