Mary Lou Pardue

   Her research has been driven largely by a desire to understand how various structural features of eukaryotic chromosomes act in the complex activities of the nucleus. In her graduate studies with J.G. Gall she developed the technique of in situ hybridization to use nuclear localization as a clue to the significance of the highly repeated DNA sequences being found in eukaryotes.  She found that different families of these sequences were localized at specific chromosomal sites, e.g. mouse satellite DNA around the centromere of each chromosome (1), Xenopus 5S RNA genes near telomeres on all chromosomes (2), Drosophila chromosomes evolving the ability to dosage compensate acquire additional dinucleotide repeats, while their homologues becoming heterochromatic have increased transposable element sequences (3).

   As biologists have developed more powerful techniques, her students have extended these initial observations to deeper understanding of specific sequence families.  Studying Drosophila telomere DNA, they discovered the first transposable element (4) that has a defined role in chromosome structure. They found that Drosophila have two retrotransposable elements that transpose only to chromosome ends where they are reverse-transcribed to form head-to-tail arrays to maintain the length of the chromosomes. Remarkably, they found retrotransposon telomeres in all Drosophila species examined, showing that this mechanism evolved before the species separated. Although the repeats in Drosophila arrays are orders of magnitude larger than the repeats that telomerase reverse transcribes to form arrays onto telomeres from Tetrahymena to humans, the Drosophila mechanism is surprisingly analogous to that of telomerase: others have shown that it is affected by the same DNA damage genes that affect telomerase telomeres.

    Initially in situ hybridization was limited to repeated DNAs because, before the advent of recombinant DNA, both probe labels and ways to analyze them were weak. However, the many aligned chromatids in Drosophila polytene chromosomes present the genome in a well-organized high-copy array. Her students used these chromosomes to analyze RNA populations in different diploid Drosophila cells. The experiments were essentially primitive DNA microarrays, using polytene chromosome bands as read-out, rather than today’s chips with defined DNA sequences. Studies of RNA labeled in a Drosophila cultured cell line revealed that these cells responded to heat shock much as the polytene cells where Ritossa had detected this response by puffing of specific chromosome bands (5). Work in several labs showed that the heat shock response was the mechanism by which cells of all organisms cope with transient stress. Her students expanded their studies to identify several levels of regulation of heat shock response that were not at the level of transcription detected by polytene puffing. They discovered that one heat shock puff, hsr-omega, encoded RNA with novel characteristics: a long non-coding poly-A+ nucleus-limited RNA plus a short cytoplasmic RNA with only one tiny open reading frame that was conserved in other species (6). The recent recognition of many new classes of RNA show that hsr-omega was in fact the first lncRNA (long non-coding RNA). Many of these newer RNAs have regulatory roles. It is possible that the function she originally suggested for hsr-omega, regulation of some aspect of nucleo-cytoplasmic coordination, will prove to be correct. Other students showed regulation of some translational elongation (7) and some specific translational initiation (8) in heat shock. At the time both appeared to be unique to Drosophila heat shock but recently stoppage in translational elongation has been reported in other systems.   

1. Pardue, M.L., and Gall, J.G.Chromosomal localization of mouse satellite DNA.Science 168:1356-1358(1970).
2. Pardue, M.L.,Localization of repeated DNA sequences in Xenopus chromosomes.Cold Spring Harbor Symp. Quant. Biol. 38:475-482(1974).
3. Lowenhaupt, K., Rich, A. and Pardue, M.L.Non-random distribution of long mono- and di-nucleotide repeats in Drosophila chromosomes; correlations with dosage compensation, heterochromatin, and recombination. Mol. Cell. Biol. 9:1173-1182 (1989).
4. Pardue, M-L, and DeBaryshe, P.G. Retrotransposons that maintain chromosome ends. Proc Natl Acad Sci U S A. 108(51):20317-24. EpubAug 5 (2011).
5. Spradling, A., Penman, S. and Pardue, M.L.Analysis of DrosophilamRNA by in situ hybridization: sequences transcribed in normal and heat shocked cultured cells.Cell 4:395-404(1975).
6. Pardue, M.L., Bendena, W.G., Fini, M.E., Garbe, J.C., Hogan, N.C., and Traverse, K.L.hsr-omega, a novel gene encoded by a Drosophila heat shock puff. Biol. Bull. 179:77-86. (1990).
7. Ballinger, D., and Pardue, M.L.The control of protein synthesis during heat shock in Drosophila cells involves altered polypeptide elongation rates.Cell 33:103-114 (1983).
8. Scott, M.P., and Pardue, M.L.Translational control in lysates of Drosophila melanogaster.Proc. Natl. Acad. Sci. USA 78:3353-3357(1981).