Site-Directed Mutagenesis

[Rani Geha]

I am constructing mutants (single amino acid changes) on a gene 
cloned into a yeast expression vector. The mutated gene will then 
be transfected and expressed in yeast.

My question is: Is there a preference as to which codon to chose 
for a particular amino acid change, or can I chose any of the 
codons that code for an amino acid?

The reason I'm asking this is because a tRNA for a certain codon  
could be in low amount resulting in low expression of my gene and 
I would like to use a codon that is common in yeast.

I am also constructing a C-terminally truncated form of the gene 
by introding a stop codon near the C-terminus. Again, is there a 
preference as to which stop codon to use, i.e. TAA, TAG, or TGA?


Thanks in advance


Rani Geha