For galactose induction of expression in Saccharomyces cerevisiae, is it possible to include additional carbon sources in the media that will increase yeast growth without repressing expression from the GAL promoter?
Some researchers choose to grow yeast in medium containing 2% galactose as the sole carbon source during induction. However, yeast typically grow more quickly in induction medium containing 2% galactose plus 2% raffinose. Raffinose is a good carbon source for yeast, and unlike glucose, does not repress transcription from the GAL promoter. Raffinose is a trisaccharide of galactose, glucose and fructose linked in that order. Most yeasts can cleave the glucose-fructose bond, but not the galactose-glucose bond. Fructose is then used as a carbon source.
Answer Id: 3734
What is the ramp rate for the Applied Biosystems® StepOne™ instrument?
What are some of the common types of auxotrophic markers in yeast?
The following are commonly employed auxotrophic markers:
1) his3Δ1: Histidine requiring strain (from gene disruption) with a deletion in locus 1. The his3 denotes the disruption of the HIS3 gene. The Δ1 is a deletion that has been engineered to decrease the recombination between the incoming plasmid DNA and the chromosomal site.
2) leu2: Leucine requiring strain due to the disruption of the LEU2 gene.
3) trp1-289: Tryptophan requiring strain, developed from gene disruption and a further point mutation to decrease the recombination between the incoming plasmid DNA and the chromosomal site.
4) ura3-52: Uracil requiring.
For more detail on types and methods of gene disruption in yeast refer to METHODS IN ENZYMOLOGY Vol. 194.
Answer Id: 3770
What are the expression levels observed from pYES2?
The following relative expression levels were observed in beta-galactosidase expression assays:
0.1 units when repressed in the presence of glucose.
Greater than 2000 units when induced in the absence of glucose and the presence of galactose.
Answer Id: 3772
Does the 2μ origin contain genes 1, 2, and 3 or just 3?
The 2μ sequence in pYES2 has the replication origin only--not sequences that code for the proteins required for self replication. Therefore, vectors such as pYES2 will replicate to high copy number (30 per cell) and are very stable in circle-plus (have endogenous 2μ) strains but are maintained only at low copy number (2 per cell) and are very unstable in circle-zero strains, with no endogenous 2μ.
Answer Id: 3773
Where is the polyadenylation sequence in pYES2?
Does pYES2 have a CEN sequence?
pYES2 does not have a CEN sequence. pYES2 has the 2μ ori which maintains a copy number of 30 to 50 copies per cell. The 2μ ori sequence is derived from the endogenous 2μ circle plasmid (it's the replication origin for 2μ circle).
'CEN' stands for 'CENtromere' sequence. This origin of replication keeps the copy number of that vector down to one or two per cell in yeast. CEN sequences are actual chromosome centromere sequences. CEN ori will keep copy number to 1 or 2 per cell (depending on whether the cell is haploid or diploid). This sequence allows the cell to recognize this plasmid as a chromosome; regulation of chromosome number is extremely rigid. Only functional CEN sequences from S. cerevisiae have been isolated and used on plasmids. CEN sequences are only a couple of hundred base pairs in size. Apparently functional centromere sequences from other eukaryotes, including Sc. pombe and Pichia, are too large to be isolated and utilized on a plasmid.
Answer Id: 3775
What could explain bands of unexpected molecular weight appearing in galactose induced pYES transformants?
Although there are no other genes on the plasmid that are induced, there certainly are a number of proteins in the cell that are turned on by galactose and any of those may be apparent. As with any experiment, one should always run the plasmid without insert, induced with galactose as a negative control. The additional bands could be due to post-translational modifications, such as glycosylation, that would increase the apparent molecular weight. Since the size of the sugar chains can be variable, glycosylated proteins often appear as less well defined bands compared to non-glycosylated proteins.
Answer Id: 3776
What is an appropriate innoculum amount to begin a galactose induction experiment? How does raffinose affect the time course of galactose induction?
The suggested initial cell density for galactose induction is 1 to 5 X 10E6 cells/ml . The cells are allowed to divide one or two times and then induced with galactose. Galactose induction is best in log phase and the culture will probably approach static phase at 1 to 4 X 10E7 cells/ml. Induction of cells maintained in raffinose may begin in 15 to 30 minutes whereas induction of cells maintained in glucose may not first occur for an hour or more. Peak expression will often occur in 2 - 4 hours so time points should be taken every hour (or every other hour) for up to 10 hours. When using raffinose maintained cells, the induction is much faster than induction of glucose maintained cells. Maximal expression levels remain the same.
Answer Id: 3778
Should D-raffinose be used as carbon source for yeast prior to galactose induction? Does L-raffinose work?
Is it critical that one uses PEG 4000 for yeast transformations?
How do I store yeast long term? Can I freeze them at –80°C like bacteria? What is the shelf life of frozen stocks?
How many copies of plasmid per yeast cell are maintained from plasmids with a 2 micron origin of replication?
What is the doubling time of pYES2 transformed S. cerevisiae strain on minimal yeast growth media when either glucose or galactose is used as the carbon source?
What are the effects of glucose on the GAL1 promoter in the pYES2 vector?