Mutations Influencing the Assimilation of Nitrogen by Yersinia pestis

Cells of 20 isolates of Yersinia (Pasteurella) pestis exhibited an unusual nutritional requirement which could be fulfilled by glycine or l-threonine. Meiotrophic mutants which required neither of these amino acids (Gly/Thr(+)) were isolated from cultures of all 20 strains at a frequency of 10(-7). Wild-type and Gly/Thr(+) cells of 14 strains failed to utilize l-amino acids or urea (0.01 m) as primary sources of nitrogen and grew slowly in the presence of low concentrations of NH(4) (+) (<== 5 mm). Cells of six strains (termed N(+)) utilized certain l-amino acids and urea (0.01 m) as primary sources of nitrogen and grew rapidly in the presence of <== 5 mm NH(4) (+). N(+) but not N(-) organisms cultivated with NH(4) (+) (0.01 m) as a primary source of nitrogen excreted a complete spectrum of naturally occurring amino acids; under this condition of growth the aspartase and particulate nicotinamide adenine dinucleotide phosphate transhydrogenase activities of N(+) and N(-) cells were repressed. N(+) meiotrophs arose at a frequency of 10(-6) in cultures of all 14 N(-) isolates, and urease-positive meiotrophs could be selected at a frequency of 10(-7) from N(+) but not N(-) cells of all 20 strains on a medium containing urea (0.01 m) as a primary source of nitrogen. These findings illustrate a reversible loss of genetic potential which has occurred during the evolution of Y. pestis as an obligate parasite and suggest that this organism is unable to efficiently remove dispensable deoxyribonucleic acid from its chromosome.