Evaluation of the Effects of PII Deficiency and the Toxicity of PipX on Growth Characteristics of the PII-Less Mutant of the Cyanobacterium Synechococcus elongatus

Among the known functions of the P-II protein (the glnB gene product) in the cyanobacterium Synechococcus elongatus, negative regulation of the activity of PipX, a transcriptional co-activator of the NtcA regulon, has been thought to be essential for cell viability, because all the P-II-less mutants thus far constructed carry spontaneous mutations in pipX. PipX is thus deduced to be a toxic protein, but its toxicity has not been clearly defined because of the lack of P-II-deficient mutants carrying wild-type pipX. In this study, we developed a method to construct a targeted P-II-less mutant of S. elongatus without the pipX mutation and determined the contribution of PipX to the detrimental effects of P-II deficiency. Growth defects of the mutant were severe under nitrogen-replete conditions, i.e. in the presence of ammonium, but were also apparent under nitrogen-limited conditions. Genetic analyses indicated that the growth impairment observed under the nitrogen-limited conditions is largely due to the toxicity of PipX. Some of the phenotypes observed under the nitrogen-replete conditions, including reduced pigmentation and death of most of the cells after transfer from nitrogen-limited conditions to nitrogen-replete conditions, were ascribed to the toxicity of PipX, but inactivation of pipX only partially rescued the growth defect observed in the presence of ammonium, indicating the presence of an as yet unknown P-II function(s) required for normal growth. Effects of ammonium addition on the nitrite uptake activity of the glnB mutant revealed a new function for P-II in regulation of the activity of the ABC-type cyanate/nitrite transporter.