Expression of proteins in yeast is a common alternative to prokaryotic and higher eukaryotic expression.What is yeast,This article will introduce yeast.
Saccharomyces cerevisiae (baker’s yeast) and Schizosaccharomyces pombe (fission yeast)
are often considered to be model eukaryotic organisms, in a manner analogous to
Escherichia coli as a model prokaryotic organism. Both yeasts have been extensively
characterized and their genomes completely sequenced. They are as easy to grow as other
microorganisms, and they have a haploid nuclear DNA content only 3.5 times that of E.
coli. However, despite the small genomes sizes, these yeasts display most of the features
of higher eukaryotes. The fact that many cellular processes are conserved among different
eukaryotic species- combined with the powerful genetic and molecular tools that are
available- has made these yeasts important experimental organisms for a variety of basic
problems in eukaryotic molecular biology.
Primarily for historical reasons, most studies on yeast have involved Saccharomyces
cerevisiae (hereafter termed yeast). Culturing yeast is simple, economical, and rapid,
characterized by a doubling time of ∼90 min on rich medium. In addition, yeast has been
well adapted to both aerobic and anaerobic large-scale culture. Cells divide mitotically
by forming a bud, which pinches off to form a daughter cell. The progression through the
cell cycle can be monitored by the size of the bud; this has been used to isolate a large
collection of mutants (called cdc mutants) that are blocked at various stages of the cell
cycle. Since yeast can be grown on a completely defined medium, many
nutritional auxotrophs have been isolated. This has not only permitted the analysis of
complex metabolic pathways but has also provided a large number of mutations useful
for genetic analysis.
Yeast can exist stably in either haploid or diploid states. A haploid cell can be either of
two mating types, called a and α. Diploid a/α cells—formed by fusion of an α cell and
an a cell ——can grow mitotically indefinitely, but under conditions of carbon and
nitrogen starvation will undergo meiosis. The meiotic products, called spores, are contained in a structure called an ascus. After gentle enzymatic digestion of the thick cell
wall of the ascus, the haploid spore products can be individually isolated and analyzed
. This ability to recover all four products of meiosis has allowed detailed genetic
studies of recombination and gene conversion that are not possible in most other
eukaryotic organisms. The existence of stable haploid and diploid states also facilitates
classical mutational analysis, such as complementation tests and identification of both
dominant and recessive mutations.
The haploid yeast cell has a genome size of about 15 megabases and contains 16 linear
chromosomes, ranging in size from 200 to 2200 kb. Thus, the largest yeast chromosome
is still 100 times smaller than the average mammalian chromosome. This small chromosome size, combined with the advent of techniques for cloning yeast genes as well as
manipulating yeast chromosomes, has allowed detailed studies of chromosome structure.
Three types of structural elements required for yeast chromosome function have been
identified and cloned: origins of replication, centromeres,
and telomeres. The cloning of these elements has led to the construction of artificial
chromosomes that can be used to study various aspects of chromosome behavior, such as
how chromosomes pair and segregate from each other during mitosis and meiosis. In
addition, systems using artificial chromosomes have been designed that allow cloning of larger contiguous segments of DNA (up to 400 kb) than are obtainable in other cloning
systems. These structural elements, as well as cloned selectable yeast genes, have
permitted the construction of yeast/E. coli shuttle vectors that can be maintained in yeast
as well as in E. coli.
Procedures for high-efficiency transformation of yeast have been available for
nearly two decades, allowing cloning of genes by genetic complementation. Because yeast has a highly efficient recombination system, DNAs with alterations
in cloned genes can be reintroduced into the chromosome at the corresponding homologous sites. This has permitted the rapid identification of the phenotypic
consequences of a mutation in any cloned gene, a technique generally unavailable in
higher eukaryotes. In addition, homologous recombination permits a wide variety of
genetic techniques that have greatly facilitated the analysis of biological processes.
Despite its small genome size, yeast is a characteristic eukaryote, containing all the major
membrane-bound subcellular organelles found in higher eukaryotes, as well as a cytoskeleton. Yeast DNA is found within a nucleus and nucleosome organization of
chromosomal DNA is similar to that of higher eukaryotes, although no histone H1 is
present. Three different RNA polymerases transcribe yeast DNA, and yeast mRNAs
(transcribed by polymerase II) show characteristic modifications of eukaryotic mRNAs
[such as a 5′ methyl-G cap and a 3′ poly(A) tail], although only a few S. cerevisiae genes
contain introns. Transcriptional regulation has been extensively studied and at least one
yeast transcriptional activator has been shown to function in higher eukaryotes as well.
High-molecular-weight yeast DNA and RNA can be prepared fairly quickly. Another characteristic of eukaryotes is the proteolytic processing of precursor
proteins to yield functional products, which is often coupled to secretion. Yeast has several
well-studied examples of secreted proteins and pheromones, and the large number of
genes that have been identified as involved in protease processing and secretion suggests
a highly complex pathway. Yeast protein extracts can be prepared using three different
protocols; the best choice will depend on the particular application. The ease
and power of genetic manipulation in yeast facilitate the use of this organism to detect
novel interacting proteins using the two-hybrid system or interaction trap.
Although Saccharomyces cerevisiae is the most commonly studied yeast, S. pombe is also
an important experimental organism. Although both yeasts are unicellular
microorganisms that grow in similar medium, they are evolutionarily quite distant. It has
become increasingly clear that, in terms of molecular mechanisms, S. pombe is more
similar to higher eukaryotic organisms than S. cerevisiae. Experimental manipulations in
S. pombe are broadly similar to those in S. cerevisiae, although the technical details often
differ. The chapter includes units on S. pombe relating to strain maintenance and media
, growth and genetic manipulation, and introduction of DNA into
cells.
Related reading: Preparation Of Yeast Media Protein Tags:How To Choose?