Studies on Vitamin B6 - Requiring Mutants of Neurospora crassa

Citation

Strauss, Bernard Samuel (1950) Studies on Vitamin B6 - Requiring Mutants of Neurospora crassa. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/1nem-3155. https://resolver.caltech.edu/CaltechTHESIS:06242025-173859797

Abstract

1. Mutants of Neurospora crassa requiring vitamin B6 have been investigated.

2. As reported by Stokes et al. (1943) certain of these mutants will grow in the absence of added vitamin B6 if the pH of the medium is above 6 and if ammonium ion is present.

3. The requirement for high pH and ammonium ion has been found to be due to a requirement for free ammonia in the medium. The role of pH is to determine the concentration of free ammonia.

4. Ammonia is specific for the initiation of growth of the pH sensitive mutants in the absence of added vitamin B6.

5. When grown in the presence of sufficient ammonia the pH sensitive mutants are able to synthesize vitamin B6.

6. The net rate of vitamin B6 synthesis is lower in the pH sensitive mutants than in wild type.

7. A glutamic-alanine transaminase has been found in Neurospora. The activity of this enzyme is lower in the mycelium of the pH sensitive mutants grown without added vitamin B6 than in wild type mycelium.

8. Resting mycelium of a pH sensitive mutant will destroy more vitamin B6 in a given period than resting mycelium of a mutant which requires vitamin B6 under all conditions tested.

9. The pH sensitive mutant is inhibited by methionine. This inhibition is overcome by added ammonia, vitamin B6 or sulfanilamide and is competitively overcome by threonine.

10. When either mutant or wild type is grown on a mixture of nitrate and ammonium as a nitrogen source at pH 7 ammonium is assimilated before nitrate.

11. Nitrate can be reduced by pH sensitive mutants grown in the absence of vitamin B6 but the product of reduction cannot be used for growth.

12. It is likely that the lower net rate of B6 synthesis in the pH sensitive mutants is due to a higher rate of vitamin B6 destruction than is shown by wild type.

13. A hypothesis to explain the specificity of ammonia has been developed. According to this idea ammonia in high concentration (relative to wild type requirements) prevents the destruction of vitamin B6.

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