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هیستیدین
Many S. cerevisiae selection schemes rely on mutant host strains that require a particular amino acid (histidine, tryptophan, or leucine) or nucleotide (uracil) for growth.
The HIS4 gene encodes a functional histidinol dehydrogenase of the histidine biosynthesis pathway.
To avoid the problems of plasmid instability during longterm growth, most P. pastoris vectors are designed to be integrated into the host genome, often within the AOX1 gene or the HIS4 gene for histidine biosynthesis.
Selective agent Action of selective agent Marker gene Action of marker gene protein Xyl-A Damages DNA Adenine deaminase (ada) Deaminates Xyl-A Blasticidin S Inhibits protein synthesis Blasticidin S deaminases (Bsr, BSD) Deaminates blasticidin S Bleomycin Breaks DNA strands Bleomycin-binding protein (Ble) Binds to bleomycin G-418 (Geneticin) Inhibits protein synthesis Neomycin phosphotransferase (neo) Phosphorylates G-418 Histidinol Produces cytotoxic efects Histidinol dehydrogenase (hisD) Oxidizes histidinol to histidine Hygromycin B Inhibits protein synthesis Hygromycin B phosphotransferase (Hph) Phosphorylates hygromycin B MSX Inhibits glutamine synthesis Glutamine synthetase (GS) Cells that produce excess glutamine synthetase survive MTX Inhibits DNA synthesis Dihydrofolate reductase (dhfr) Cells that produce excess dihydrofolate reductase survive PALA Inhibits purine synthesis Cytosine deaminase (codA) Lowers cytosine levels in the medium by converting cytosine to uracil Puromycin Inhibits protein synthesis Puromycin N-acetyltransferase (Pac) Acetylates puromycin MSX, methionine sulfoximine; MTX, methotrexate; PALA, N(phosphoacetyl)laspartate; XylA, 9کdxylofuranosyl adenine.
(C) To fur ther increase acidstability, four histidines (H257, H259, H268, H290) that are pro tonated at low pH were changed to phenylalanine (F257, F259, F268, F290).
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