Ultrabithorax

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.[1]

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.[2]

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.[3]

Ubx represses selected Dpp target genes in the anterior and posterior axis.[4] 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.[5][6]

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.[7] 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.[8]

References

  1. ↑ White RA, Lehmann R (October 1986). "A gap gene, hunchback, regulates the spatial expression of Ultrabithorax". Cell. 47 (2): 311–321. doi:10.1016/0092-8674(86)90453-8. PMID 2876779.
  2. ↑ "FlyBase Gene Report: Dmel\Ubx". FlyBase. March 20, 2009. Retrieved 2009-04-23.
  3. ↑ Pavlopoulos A, Akam M (2011). "Hox gene Ultrabithorax regulates distinct sets of target genes at successive stages of Drosophila haltere morphogenesis". Proc Natl Acad Sci U S A. 108 (7): 2855–2860. doi:10.1073/pnas.1015077108. PMC 3041078Freely accessible. PMID 21282633.
  4. ↑ Capovilla M, Brandt M, Botas J (February 1994). "Direct regulation of decapentaplegic by Ultrabithorax and its role in Drosophila midgut morphogenesis". Cell. 76 (3): 461–475. doi:10.1016/0092-8674(94)90111-2. PMID 7906203.
  5. ↑ Petruk S, Sedkov Y, Riley KM, et al. (December 2006). "Transcription of bxd noncoding RNAs promoted by trithorax represses Ubx in cis by transcriptional interference". Cell. 127 (6): 1209–21. doi:10.1016/j.cell.2006.10.039. PMC 1866366Freely accessible. PMID 17174895.
  6. ↑ Petruk S, Sedkov Y, Brock HW, Mazo A (2007). "A model for initiation of mosaic HOX gene expression patterns by non-coding RNAs in early embryos". RNA Biol. 4 (1): 1–6. doi:10.4161/rna.4.1.4300. PMID 17568198.
  7. ↑ Greer AM, Huang Z, Oriakhi A, Lu Y, Lou J, Matthews KS, Bondos SE (2009). "The Drosophila transcription factor ultrabithorax self-assembles into protein-based biomaterials with multiple morphologies". Biomacromolecules. 10 (4): 829–837. doi:10.1021/bm801315v. PMID 19296655.
  8. ↑ Huang, Z.; Lu, Y.; Majithia, R.; Shah, J.; Meissner, K.; Matthews, K. S.; Bondos, S. E.; Lou, J. (2010). "Size Dictates Mechanical Properties for Fibers Self-Assembled by the Drosophila Hox Transcription Factor Ultrabithorax". Biomacromolecules. 11 (12): 3644–3651. doi:10.1021/bm1010992. PMID 21047055.

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