Developmentally regulated transcription factors in Drosophila melanogaster

During early stages of Drosophila development the heat shock response cannot be induced. It is reasoned that the adverse effects on cell cycle and cell growth brought about by Hsp70 induction must outweigh the beneficial aspects of Hsp70 induction in the early embryo. Although the Drosophila heat shock transcription factor (dHSF) is abundant in the early embryo, it does not enter the nucleus in response to heat shock. In older embryos and in cultured cells the factor is localized within the nucleus in an apparent trimeric structure that binds DNA with high affinity. The domain responsible for nuclear localization upon stress resides between residues 390 and 420 of the dHSF. Using that domain as bait in a yeast two-hybrid system we now report the identification and cloning of a nuclear transport protein Drosophila karyopherin-α3(dKap- α3). Biochemical methods demonstrate that the dKap-α3 protein binds specifically to the dHSF's nuclear localization sequence (NLS). Furthermore, the dKap-α3 protein does not associate with NLSs that contain point mutations which are not transported in vivo. Nuclear docking studies also demonstrate specific nuclear targeting of the NLS substrate by dKap-α3.Consistant with previous studies demonstrating that early Drosophila embryos are refractory to heat shock as a result of dHSF nuclear exclusion, we demonstrate that the early embryo is deficient in dKap-α3 protein through cycle 12. From cycle 13 onward the transport factor is present and the dHSF is localized within the nucleus thus allowing the embryo to respond to heat shock.

The pair-rule gene fushi tarazu (ftz) is a well-studied zygotic segmentation gene that is necessary for the development of the even-numbered parasegments in Drosophila melanogastor. During early embryogenesis, ftz is expressed in a characteristic pattern of seven stripes, one in each of the even-numbered parasegments. With a view to understand how ftz is transcriptionally regulated, cDNAs that encode transcription factors that bind to the zebra element of the ftz promoter have been cloned. Chapter Ill reports the cloning and characterization of the eDNA encoding zeb-1 (zebra element binding protein), a novel steroid receptor-like molecule that specifically binds to a key regulatory element of the ftz promoter. In transient transfection assays employing Drosophila tissue culture cells, it has been shown that zeb-1 as well as a truncated zeb-1 polypeptide (zeb480) that lacks the putative ligand binding domain function as sequencespecific trans-activators of the ftz gene.

The Oct factors are members of the POU family of transcription factors that are shown to play important roles during development in mammals. Chapter IV reports the eDNA cloning and expression of a Drosophila Oct transcription factor. Whole mount in-situ hybridization experiments revealed that the spatial expression patterns of this gene during embryonic development have not yet been observed for any other gene. In early embryogenesis, its transcripts are transiently expressed as a wide uniform band from 20-40% of the egg length, very similar to that of gap genes. This pattern progressively resolves into a series of narrower stripes followed by expression in fourteen stripes. Subsequently, transcripts from this gene are expressed in the central nervous system and the brain. When expressed in the yeast Saccharomyces cerevisiae, this Drosophila factor functions as a strong, octamer-dependent activator of transcription. The data strongly suggest possible functions for the Oct factor in pattern formation in Drosophila that might transcend the boundaries of genetically defined segmentation genes.

A fundamental approach to phase noise reduction in hybrid Si/III-V lasers

Spontaneous emission into the lasing mode fundamentally limits laser linewidths. Reducing cavity losses provides two benefits to linewidth: (1) fewer excited carriers are needed to reach threshold, resulting in less phase-corrupting spontaneous emission into the laser mode, and (2) more photons are stored in the laser cavity, such that each individual spontaneous emission event disturbs the phase of the field less. Strong optical absorption in III-V materials causes high losses, preventing currently-available semiconductor lasers from achieving ultra-narrow linewidths. This absorption is a natural consequence of the compromise between efficient electrical and efficient optical performance in a semiconductor laser. Some of the III-V layers must be heavily doped in order to funnel excited carriers into the active region, which has the side effect of making the material strongly absorbing.

This thesis presents a new technique, called modal engineering, to remove modal energy from the lossy region and store it in an adjacent low-loss material, thereby reducing overall optical absorption. A quantum mechanical analysis of modal engineering shows that modal gain and spontaneous emission rate into the laser mode are both proportional to the normalized intensity of that mode at the active region. If optical absorption near the active region dominates the total losses of the laser cavity, shifting modal energy from the lossy region to the low-loss region will reduce modal gain, total loss, and the spontaneous emission rate into the mode by the same factor, so that linewidth decreases while the threshold inversion remains constant. The total spontaneous emission rate into all other modes is unchanged.

Modal engineering is demonstrated using the Si/III-V platform, in which light is generated in the III-V material and stored in the low-loss silicon material. The silicon is patterned as a high-Q resonator to minimize all sources of loss. Fabricated lasers employing modal engineering to concentrate light in silicon demonstrate linewidths at least 5 times smaller than lasers without modal engineering at the same pump level above threshold, while maintaining the same thresholds.