FIG. 6.

In vivo phosphorylation of the P_II protein in cells of wild-type Synechococcus sp. strain PCC 7942 (WT) and the NtcA-deficient mutant (NtcA⁻) in response to the nature of the nitrogen source and CO₂ availability. Lanes 1 to 3, the cells were incubated as described in the legend to Fig. 2. Lanes 4, cells starved for nitrogen as in lanes 3 were further incubated for 2 h following addition of ammonium to the culture medium. P_II⁰, P_II¹, P_II², and P_II³ correspond to the four isoforms of the trimeric P_II protein containing zero, one, two, or three phosphorylated monomers.