Solid-state NMR studies of Klebsiella pneumoniae grown under nitrogen-fixing conditions

The carbon and nitrogen metabolism of Klebsiella pneumoniae M5a1 has been characterized using ¹³C and ¹⁵N labeling with detection by cross-polarization magic-angle spinning solid-state NMR. Cells grown on ammonium typically require some 20 h to derepress fully for nitrogenase when transferred to medium devoid of any source of fixed nitrogen. We have established that during this period some cellular proteins are catabolized with the liberated nitrogen being used for the synthesis of purines needed for formation of ribosomal RNA. The 20-h derepression period can be shortened to 6 h by the introduction of fixed nitrogen in certain specific forms. Serine is the most successful agent we have examined for shortening the derepression period and glycine among the least successful. We attribute this difference to the advantage of serine over glycine in providing both specific and nonspecific carbon and nitrogen sources for complete purine synthesis. These determinations were made by tracing the metabolism of ¹³C- and ¹⁵N-labeled chemical bonds from the 2 amino acids during derepression.