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fusB is an allele of nadD, encoding nicotinate mononucleotide adenylyltransferase in Escherichia coli

Department of Microbiology, Stockholm University, S-106 91 Stockholm, Sweden

Correspondence
Monica Rydén-Aulin
monica{at}mibi.su.se

Isolation of the temperature-sensitive Escherichia coli mutant 72c has been described previously. The mutant allele was named fusB and causes a pleiotropic phenotype, the most striking features of which, besides temperature sensitivity, are the inability to grow on synthetic medium and supersensitivity to trimethoprim, an antibiotic that inhibits the C₁ metabolism. This work shows that the fusB mutation is a frameshift mutation in the nadD gene that encodes nicotinate mononucleotide adenylyltransferase. The frameshift leads to a change of the last 10 amino acids and an addition of 17 amino acids. This lesion, renamed nadD72, leads to very little NAD⁺ and NADPH synthesis at the permissive temperature and essentially no synthesis at the non-permissive temperature. As a comparison, a new mutation in the nadD gene, with an amino acid change in the ATP-binding site, has been isolated. Its NAD⁺ synthesis is decreased at 30 °C but the level is still sufficient to support normal growth. At 42 °C, NAD⁺ synthesis is reduced further, which leads to temperature sensitivity on minimal medium. This mutation was designated nadD74. Thus, a small decrease in NAD⁺ levels affects ability to grow on minimal medium at 42 °C, while a large decrease leads to a more pleiotropic phenotype.

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