Figure 2. In vivo expression of nif'-'lacZ from the nifE (E) and nifH (H) promoters in the E. coli MC1061 strain in the presence (+ NifA) or absence of the NifA activator protein provided by pCK3 plasmid. Expression of nif'-'lacZ fusion was measured as a function of the OD₆₀₀ for cultures grown in A medium (see Material and Methods). In each case, three data points (between OD₆₀₀ of 0.2 and 0.6) were used to determine the slope of the line, which reflects the differential rate of nif'-'lacZ expression. Curve labeled MC (which overlaps the curve from the nifH promoter in the absence of NifA) gives data for the E. coli MC1061 host strain (negative control).