Physiological role of the GlnK signal transduction protein of Escherichia coli : survival of nitrogen starvation

Summary Escherichia colicontains two PII‐like signal trans‐duction proteins, PII and GlnK, involved in nitrogen assimilation. We examined the roles of PII and GlnK in controlling expression ofglnALG,glnKandnacduring the transition from growth on ammonia to nitrogen starvation and vice versa. The PII protein exclusively controlledglnALGexpression in cells adapted to growth on ammonia, but was unable to limitnacandglnKexpression under conditions of nitrogen starvation. Conversely, GlnK was unable to limitglnALGexpression in cells adapted to growth on ammonia, but was required to limit expression of theglnKandnacpromoters during nitrogen starvation. In the absence of GlnK, very high expression of theglnKandnacpromoters occurred in nitrogen‐starved cells, and the cells did not reduceglnKandnacexpression when given ammonia. Thus, one specific role of GlnK is to regulate the expression of Ntr genes during nitrogen starvation. GlnK also had a dramatic effect on the ability of cells to survive nitrogen starvation and resume rapid growth when fed ammonia. After being nitrogen starved for as little as 10 h, cells lacking GlnK were unable to resume rapid growth when given ammonia. In contrast, wild‐type cells that were starved immediately resumed rapid growth when fed ammonia. Cells lacking GlnK also showed faster loss of viability during extended nitrogen starvation relative to wild‐type cells. This complex phenotype resulted partly from the requirement for GlnK to regulatenacexpression; deletion ofnacrestored wild‐type growth rates after ammonia starvation and refeeding to cells lacking GlnK, but did not improve viability during nitrogen starvation. The specific roles of GlnK during nitrogen starvation were not the result of a distinct function of the protein, as expression of PII from theglnK promoter in cells lacking GlnK restored the wild‐type phenotypes.