An alternative P-II protein in the regulation of glutamine synthetase in Escherichia coli

The P-II protein has been considered pivotal to the dual cascade regulating ammonia assimilation through glutamine synthetase activity. Here we show that P-II, encoded by the glnB gene, is not always essential; for instance upon ammonia deprivation of a glnB deletion strain, glutamine synthetase can be deadenylylated as effectively as in the wild-type strain. We describe a new operon, glnK amtB, which encodes a homologue of P-II and a putative ammonia transporter. We cloned and overexpressed glnK and found that the expressed protein had almost the same molecular weight as P-II, reacted with polyclonal P-II antibody, and was 67% identical in terms of amino acid sequence with Escherichia coli P-II. Like P-II, purified GlnK can activate the adenylylation of glutamine synthetase in vitro, and, in vivo, the GlnK protein is uridylylated in a glnD-dependent fashion. Unlike P-II, however, the expression of glnK depends on the presence of UTase, nitrogen regulator I (NRI), and absence of ammonia. Because of a NRI and a sigma(N) (sigma(54)) RNA polymerase-binding consensus sequence upstream from the glnK gene, this suggests that glnK is regulated through the NRI/NRII two-component regulatory system. Indeed, in cells grown in the presence of ammonia, glutamine synthetase deadenylylation upon ammonia depletion depended on P-II, Possible regulatory implications of this conditional redundancy of P-II are discussed.