Biology Reference
In-Depth Information
6.1
Introduction
The transmission of defined visible traits, that is, phenotypes from
one generation to the next, is a central aspect in living organisms,
and the nucleic acid sequence units that constitute the genes are the
stable entities of inheritance. While the appearance of new traits
has long been considered as the sole consequence of mutations
in nucleic acid sequences, studies from the past two decades have
proven that new traits can be passed between individuals without
changes in nucleic acid sequences.
The revolutionary idea that some proteins could behave as
infectious agents carrying alone the hereditary information that
ensures their propagation was first proposed by the mathematician
Griffith.
1
Such proteins are now termed prions for “infectious
proteins”
2
and their ability to transmit novel phenotypes is believed
to be the consequence of their capacity to adopt more than one
stable conformation. Indeed, since the structure of a polypeptide
defines its function, cells containing the same polypeptide chain in
different conformations will have distinct phenotypes. However, if
newly synthesized prion molecules adopt in an independent manner
a wide range of different conformations, all cells would exhibit the
same phenotype reflecting an average functional state. In contrast,
if the conformation of such proteins is constrained in different cells,
distinctive phenotypes will be displayed.
6.2
Prions in Yeast
+
In the baker's yeast,
Saccharomyces cerevisiae
, three traits, [PSI
],
+
[URE3], and [PIN
], and in the filamentous fungus, (
Podospora
anserine
), one feature, [Het-s], are inherited in a non-Mendelian
3-6
manner.
It was first thought that these traits were due to a non-
chromosomal nucleic acid. However, [PSI
+
+
], and
[Het-s] inheritance differs significantly from that of DNA plasmids,
RNA viruses, the mitochondrial genome, or RNA replicons. In
addition, the genes encoding these traits are located in the nucleus
of yeast cells.
], [URE3], [PIN
This led to the idea that these traits are due to the
prion properties of the proteins Sup35, Ure2, Rnq1, and HET-s,
respectively.
6-8
9
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