BEADLE AND TATUM 



of a commercial concentrate contain- 

 ing 100 micrograms of biotin per cc.** 

 Any loss of ability to synthesize an es- 

 sential substance present in the com- 

 plete medium and absent in the minimal 

 medium is indicated by a strain grow- 

 ing on the first and failing to grow on 

 the second medium. Such strains are 

 then tested in a systematic manner to 

 determine what substance or sub- 

 stances they are unable to synthesize. 

 These subsequent tests include at- 

 tempts to grow mutant strains on the 

 minimal medium with ( 1 ) known vita- 

 mins added, (2) amino acids added or 

 (3) glucose substituted for the more 

 complex carbon source of the minimal 

 medium. 



Single ascospore strains are individ- 

 ually derived from perithecia of N. 

 crassa and N. sitophila x-rayed prior 

 to meiosis. Among approximately 2000 

 such strains, three mutants have been 

 found that grow essentially^ normally 

 on the complete medium and scarcely 

 at all on the minimal medium with 

 sucrose as the carbon source. One of 

 these strains {N. sitophila) proved to 

 be unable to synthesize vitamin Be 

 (pyridoxine). A second strain {N. 

 sitophila) turned out to be unable to 

 synthesize vitamin Bi (thiamine). Ad- 

 ditional tests show that this strain is 

 able to synthesize the pyrimidine half 

 of the Bi molecule but not the thiazole 

 half. If thiazole alone is added to the 

 minimal medium, the strain grows es- 

 sentially normally. A third strain {N. 

 crassa) has been found to be unable to 

 synthesize para-aminobenzoic acid. 

 This mutant strain appears to be en- 

 tirely normal when grown on the 

 minimal medium to which p-amino- 

 benzoic acid has been added. Only in 

 the case of the "pyridoxinless" strain 

 has an analysis of the inheritance of 



* The biotin concentrate used was ob- 

 tained from the S. M. A. Corporation, Cha- 

 grin Falls, Ohio. 



169 



the induced metabolic defect been in- 

 vestigated. For this reason detailed 

 accounts of the thiamine-deficient and 

 p-aminobenzoic acid-deficient strains 

 will be deferred. 



Qualitative studies indicate clearly 

 that the pyridoxinless mutant, grown 

 on a medium containing one micro- 

 gram or more of synthetic vitamin Be 

 hydrochloride per 25 cc. of medium, 

 closely approaches in rate and char- 

 acteristics of growth normal strains 

 grown on a similar medium with no 

 Be. Lower concentrations of Be give 

 intermediate growth rates. A prelimi- 

 nary investigation of the quantitative 

 dependence of growth of the mutant 

 on vitamin Be in the medium gave the 

 results summarized in table 1. Addi- 

 tional experiments have given results 

 essentially similar but in only approxi- 



Table 1 



Growth of Pyridoxinless Strain of N. sito- 

 phila on Liquid Medium Containing Inor- 

 ganic Salts,» r/o Sucrose, and 0.004 Micro- 

 gram Biotin per Cc. Temperature 25 °C. 

 Growth Period, 6 Days from Inoculation 

 with Conidia 



9 Throughout our work with Neurospora, 

 we have used as a salt mixture the one 

 designated number 3 by Fries, N., Symbolae 

 Bot. Upsalienses, vol. 3, No. 2, 1-188, 1938. 

 This has the following composition: NH^ 

 tartrate, 5 g.; NH4NO3, 1 g.; KH0PO4, 1 g.; 

 MgS04-7HoO, 0.5 g.l NaCl, O.l'g.; CaCU, 

 0.1 g.; FeCl3,''lO drops 1% solution; HoO, 1 1. 

 The tartrate cannot be used as a carbon 

 source by Neurospora. 



