Auxotrophy (Gr. ?????? "to increase"; ????? "nourishment") is the inability of an organism to synthesize a particular organic compound required for its growth (as defined by IUPAC). An auxotroph is an organism that displays this characteristic; auxotrophic is the corresponding adjective. Auxotrophy is the opposite of prototrophy, which is characterized by the ability to synthesize all the compounds needed for growth. The method of replica plating implemented by Esther Lederberg included auxotrophs that were temperature-sensitive; that is, their ability to synthesize was temperature-dependent. (Auxotrophs are usually not temperature-dependent. They can also depend on other factors, such as light intensity or wavelength.) Multiple auxotrophs can also coexist at the same time, within the same organism.[1] In genetics, a strain is said to be auxotrophic if it carries a mutation that renders it unable to synthesize an essential compound. For example, a yeast mutant with an inactivated uracil synthesis pathway gene is a uracil auxotroph. (E.g., if the yeast Orotidine 5'-phosphate decarboxylase gene is inactivated, the resultant strain is a uracil auxotroph.) Such a strain is unable to synthesize uracil and will only be able to grow if uracil can be taken up from the environment. This is the opposite of a uracil prototroph, or in this case a wild-type strain, which can still grow in the absence of uracil. Auxotrophic genetic markers are often used in molecular genetics; they were famously used in Beadle and Tatum's Nobel prize-winning work on the one gene-one enzyme hypothesis. Researchers have used strains of E. coli auxotrophic for specific amino acids to introduce non-natural amino acid analogues into proteins. For instance cells auxotrophic for the amino acid phenylalanine can be grown in media supplemented with an analogue such as para-azido phenylalanine. Many living things, including humans, are auxotrophic for large classes of compounds required for growth and must obtain these compounds through diet (see vitamin, essential amino acid, Esther Miriam Zimmer Lederberg (December 18, 1922 November 11, 2006) was an American microbiologist and immunologist and pioneer of bacterial genetics. Notable contributions include the discovery of the lambda phage, the relationship between transduction and lambda phage lysogeny, the development of replica plating, and the discovery of bacterial fertility factor F. Lederberg also founded and directed the Plasmid Reference Center at Stanford University, whose collection contained plasmids of all types of genes, coding for antibiotic resistance, heavy metal resistance, virulence, conjugation, colicins, transposons, temperature sensitivity and other unknown factors. (Most of these plasmids have still not been thoroughly studied.)