Figure 2. Composition of the DR-ultrafiltrate.

(A) Manganese (Mn) and phosphate (PO₄) concentrations in bacterial ultrafiltrates (100%). (B) Nucleoside and base (Ns/Nb) concentrations in bacterial ultrafiltrates (100%) (Table S1). (C) Sum of free amino acids and those in peptide linkage in bacterial ultrafiltrates (100%) after acid hydrolysis. The total amino acid concentration of the DR-ultrafiltrate is 53 mM, of which 97% are in peptides (see also Figure S2). (D) Mn-complexes. The distribution of Mn bound to small molecules and peptides in aqueous-phase extracts of D. radiodurans homogenate was determined by size exclusion chromatography. The cell homogenate was prepared from cells disrupted in the presence of protease inhibitors. Note, gel filtration causes a significant dilution of the original sample, so the measured concentration in the fractions does not reflect the undiluted concentration in the sample. The 100% value for amino acids is 330 µM and that for Mn is 3.8 µM. As any unbound Mn or free amino acid-bound Mn would have eluted late in the chromatographic analysis, we concluded that Mn was bound to peptides. Proteins and large peptides >3,500 Da eluted together at the exclusion volume of the column; average molecular weight of an amino acid in peptide linkage is 115 Da.