TABLE 2.

Enzyme assay of LsdB secretion in G. diazotrophicus^a

Strain/mutant/transconjugant	Intracellular LsdB activity (U mg−1)	Extracellular LsdB activity (U mg−1)
SRT4	0.25 ± 0.2	14.24 ± 0.9
AD6	0.75 ± 0.2	0.33 ± 0.3
AD8	0.73 ± 0.1	0.52 ± 0.2
AD9	0.69 ± 0.3	0.71 ± 0.3
SRT4(pALS198)	1.97 ± 0.8	40.53 ± 2.2
AD8(pALS198)	2.75 ± 0.9	1.61 ± 0.6

^aOne unit is defined as the amount of LsdB releasing 1 μmol of fructose per min based on initial velocity measurements under the following reaction conditions: 1% levan in 0.1 M sodium acetate buffer, pH 5, at 30°C. Activity is given per mg of total soluble proteins, as measured by the Bradford procedure (8). Levan from Erwinia herbicola (Sigma) was used as the substrate. The values are means ± standard deviations (n = 5).