Gene: pyrimidine
pyrimidine
locus: pyr
locus_name: pyrimidine
organism_type: B
chromosome_number:
chromosome_side:
link_group:
cultural_requirements: All pyrimidine auxotrophs of Neurospora are nonspecific, responding to any pyrimidine nucleoside, nucleotide, or base. The symbol pyr is, therefore, used for genes concerned with the biosynthetic pathway. Nucleosides or nucleotides are more effective than corresponding bases as growth supplements for the mutant pyr-1 (623) and apparently for other pyr mutants. However, after a lag, uracil is used nearly as effectively as uridine (683). No cytidine or thymidine-specific requirement exists, because Neurospora lacks thymidine kinase (421) and because any exogenous pyrimidine supplement is cycled back through uridine monophosphate, which provides all the normal end products of pyrimidine biosynthesis (1141). For this reason, DNA cannot be specifically labeled by supplying [3H]thymidine, under normal circumstances. Mutations have been obtained (uc-2, -3, 4, -5; ud-1) that block the pathway back through uridine monophosphate and so prevent general labeling from a single precursor (1141). Cytosine in DNA can be labeled specifically by the method of Worthy and Epler (1163). For a general review of pyrimidine metabolism, see reference 766. For systematic gene-enzyme work, see references 133 and 134. For pyrimidine biosynthetic pathway, see Fig. 20. For loci concerned with pyrimidine salvage or pyrimidine uptake, see uc, ud, udk, and Fig. 23. Complex interactions between lys and pyr mutations have been described (485). Pyrimidine biosynthetic enzymes differ in their modes of regulation. The pyrimidine-specific carbamyl phosphate synthase-aspartate carbamyl transferase complex is derepressed by end product depletion, but is insensitive to repression in the fluoropyrimidine-resistant mutant fdu-2 (127, 135); dihydroorotase is relatively unresponsive to end-product limitation; and dihydroorotate dehydrogenase is induced by a precursor which is probably, by analogy with Saccharomyces, dihydroorotate, the substrate of the enzyme. Regulation of the last two enzymes has not been studied systematically. Pyrimidine regulation of the uptake and salvage pathways of pyrimidine is discussed under individual loci; see uc-5 and ud-1. Many aspects of pyrimidine metabolism are under the control of general nitrogen metabolite regulation (128).
enzyme_id:
anid_name:
near_l:
near_r:
neu_name:
supplement: pyrimidine
image_url:
Strains
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References
