Loss of gene activity leads to a nutritional requirement (auxotrophy) not exhibited by the wildtype (prototrophy).
Amino acids:
*ala = alanine
*arg = arginine
*asn = asparagine
Some pathways produce metabolites that are precursors of more than one pathway. Hence, loss of one of these enzymes will lead to a requirement for more than one amino acid. For example:
*ilv: isoleucine and valine
Nucleotides:
*gua = guanine
*pur = purines
*pyr = pyrimidine
*thy = thymine
Vitamins:
*bio = biotin
*nad = NAD
*pan = pantothenic acid