Table 1.

Isotopic labeling of GB1 samples prepared for this study

Sample	Isotopic labeling condition
A	Uniform-13C, 15N-enriched*
B	Uniform 15N†
C	1:4 ratio of uniform-13C,15N-labeled in natural abundance‡
D	Prepared from 1,3-13C-glycerol and 15NH4Cl§
E	Prepared from 2-13C-glycerol and 15NH4Cl¶

*Expressed with ¹³C glucose and ¹⁵NH₄Cl.

^†Expressed with natural abundance glucose and ¹⁵NH₄Cl.

^‡One part uniformly ¹³C,¹⁵N-labeled GB1 was diluted in four parts natural abundance GB1 prior to precipitation by the standard protocol.

^§Expressed with 1,3-¹³C-glycerol and natural abundance Na₂CO₃ as the sole carbon sources in the growth medium and ¹⁵NH₄Cl.

^¶Expressed with 2-¹³C-glycerol and Na₂¹³CO₃ as the sole carbon sources and ¹⁵NH₄Cl.