> You take a gene from a plant, change the codons to whatever yeast uses

This isn't necessary; codon usage is very basal. The biggest challenge you might face is prokaryotic vs. eukaryotic expression, but that's due to processing of the transcript (e.g. splicing) and the nascent peptide (e.g. all kinds of stuff). Messing with codons is simply optimization.

Expression is indeed the hard part, but not because of regulation. The general strategy is to use inducible promoters: grow the yeast, add chemical, get protein/metabolite.

Because this isn't constitutive, you can get away with a lot; even high levels of expression that will kill the cell will still produce enough of the desired product that it's an effective approach. And even relative gene-product dosage isn't too bad because you can associate the different gene products with particular known promoters and enhancers, etc.

What you have to account for is the general state of the cell; is it producing enough precursors, are intermediate products toxic, do they need to be confined for modification/processing, etc. etc. Doing complicated biochemistry gets messy.

Making simple peptides, e.g. insulin, is easy, and that's why it was the first GM pharmaceutical. Making a whole biochemical processing facility in a cell is .. a bit more of a challenge.

Thanks! Could you shed some more light on how one actually takes a gene from a plant (in a lab setting I suppose) and put it into yeast?

* Extract RNA

* Reverse-transcribe this into DNA

* PCR amplify target sequence using custom primers. Generally you design the primers to create restriction sites flaking the sequence of interest.

* Restriction digest the amplicons

* Pop that into a cloning vector, e.g. Gateway

* Clone and isolate a colony

* Sanger sequence to confirm you've got what you want

* Move construct into expression vector

* Transform yeast with expression vector construct

* Sequence some colonies to find one that worked well (these vectors use resistance genes to provide an easy screening process)

* Once you've confirmed it worked, you're all set.

You can just use plasmids or do a stable transformation; the latter is harder but it's what you'd want for commercial production I think.

Step 1 is you hope that someone already identified the gene(s) that do what you want. You look it up in a database and copy-paste the DNA sequence into a word document.

Step 2 you adjust the DNA sequence for the target organism. This might mean using different codons, changing the start of the gene, etc. Usually you also append a few extra genes like GFP or antibiotic resistance genes that make working with your DNA sequence easier.

Step 3 upload your DNA sequence to an online store for DNA synthesis. A few days later you'll get an envelope with a small plastic vial with your synthesized DNA.

Step 4 Now you need to get your DNA into the target organism. I'm not sure how you do that for yeast, but there's probably a well known protocol you can follow. You might also first put your DNA into bacteria to make more of it.

Step 5 now grow your cells, and put them under the microscope, check if they glow green (that's why you put the GFP in there) and perform various other analysis to see if what you did works. It probably didn't, so you go back and change something, and try again.

https://en.wikipedia.org/wiki/Illumina_dye_sequencing https://en.wikipedia.org/wiki/CRISPR