Results and Discussion

The high-speed supernatant and two different ion exchange fractions (100 and 250 mM NaCl) from R. rubrum cells grown under nitrogen-fixing conditions show adenylation activity with ATP, MgC^ and glutamine added. ADP can substitute for ATP giving almost the same decrease in GS activity. Thus, ATase may use either ADP or ATP as substrate for adenylation of GS. The rate of the adenylation is decreased by a-ketoglutarate (Jiang et al. 1998). Adenylated GS treated with snake venom phosphodiesterase shows an increase in activity, indicating conversion to the unmodified form. Deadenylation of GS in the supernatant is stimulated by Pi and a-ketoglutarate. We cannot observe deadenylation activity in the ion-exchange fractions even if PII-UMP, Pi and a-ketoglutarate are added. Thus, a component required for the deadenylation may be absent or the added PII-UMP demodified. The 250 mM NaCl fraction requires PII for modification of GS with either ATP or ADP. Western blotting with polyclonal antibodies against E. coli ATase shows that R. rubrum ATase is approximately 115 kDa the same as E. coli (Caban et al. 1976). A strong immunoreaction around 55-60 kDa in the 100 mM fraction suggest that ATase might be degraded within its Q-linker (Jaggi et al. 1997). The 100 mM fraction also contains most of the adenylation activity but no deadenylation activity.

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