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transfer RNAs (tRNAs), histone proteins, transposable elements, and developmen-
tally regulated multigene families such as actins, cuticle genes, heat-shock genes,
larval-serum genes, silk genes, and yolk-protein genes. One solution to produc-
ing large amounts of gene product in a relatively short time and in a coordinated
manner is to duplicate the gene. Duplicated genes may be present in tandem
arrays on the same chromosome or be present on separate chromosomes.
4.9.1 Heat-Shock Genes
The heat-shock response originally was discovered in D. melanogaster and has
since been found in organisms ranging from bacteria to humans. Heat-shock
genes are activated in response to environmental stresses such as heat or chemi-
cal shock. Heat-shock proteins are present in small amounts in many cells in the
absence of stress, but they rapidly increase after stress. Heat-shock genes are
an evolutionarily conserved response to stress in all organisms ( Morimoto et al.
1992 ).
If Drosophila adults are exposed to a severe heat shock ( 40°C), most die.
If they undergo a mild shock at 33°C, additional heat-shock proteins are syn-
thesized, and many flies then can survive subsequent heat shocks at 40°C. In
D. melanogaster , nine chromosomal sites puff in response to heat shock, and
specific mRNAs are produced that code for seven heat-shock proteins. There are
several types (or families), including the hsp70 , hsp83 , and the small heat-shock
gene family ( Pauli et al. 1992 ).
hsp70 is virtually inactive in unstressed cells, but hsp proteins become very
abundant during and after heat shock, accounting for 1% of the total cel-
lular protein ( Feder and Krebs 1997 ). There are 10 copies of the hsp70 gene in
Drosophila . It is the most abundant and highly conserved. At the amino-acid level,
the Drosophila hsp70 protein shares 73% overall similarity with the human and
50% with that of the bacterium Escherichia coli . In addition, seven cognate genes
of hsp70 are constitutively expressed and may be important during Drosophila
development. The hsp70 proteins are molecular chaperones, minimizing aggrega-
tion of peptides in nonnative conformation.
The hsp83 gene products are general chaperones involved in several devel-
opmental pathways in D. melanogaster and have both housekeeping and
stress-related functions ( Rutherford and Lindquist 1998, Mayer and Bukau
1999 ). When the D. melanogaster hsp83 gene is mutated or impaired, vari-
ability in many adult structures is induced, with specific variants depending on
the genetic background. Thus, hsp83 gene products buffer variation, allowing
genetic variation to accumulate under neutral conditions. When the organism
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