Two classes of pyridine nucleotide uptake mutants isolated previously in a strain of defective in both biosynthesis () and pyridine nucleotide recycling () were analysed in terms of their genetic relationship to each other and their roles in the transport of nicotinamide mononucleotide as a precursor to The first class of uptake mutants, (99 units), failed to grow on nicotinamide mononucleotide () as a precursor for The second class, , grew on lower than normal levels of and suppressed mutations. A third class of uptake mutant, , isolated in a background, also failed to grow on Transport studies and enzyme analyses confirmed these strains as defective in uptake. A fourth locus, designated , was found to diminish utilization in a background. Tn insertions near and were isolated and utilized in mapping studies. was found to map between and near The locus was cotransducible with at 17 units while mapped at approximately 60 units. The biochemical and genetic data suggest that the and gene products cooperate in the utilization of under normal conditions. A mutant, however, does not require the gene product for uptake but does rely on the product. Fusion studies indicate that is regulated by internal concentrations.


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