Phosphohistidyl active sites in polyphosphate kinase of Escherichia coli

Inorganic polyphosphate: a molecule of many functions

Pursuit of the enzymes that make and degrade polyP has provided analytic reagents which confirm the ubiquity of polyP in microbes and animals and provide reliable means for measuring very low concentrations. Many distinctive functions appear likely for polyP depending on its abundance, chain length, biologic source and subcellular location: an energy supply and ATP substitute, a reservoir for Pi, a chelator of metals, a buffer against alkali, a channel for DNA entry, a cell capsule, and, of major interest, a regulator of responses to stresses and adjustments for survival in the stationary phase of culture growth and development. Whether microbe or human, we depend on adaptations in the stationary phase, a dynamic phase of life. Much attention has focused on the early and reproductive phases of organisms, rather brief intervals of rapid growth, but more concern needs to be given to the extensive period of maturity. Survival of microbial species depends on being able to manage in the stationary phase. In view of the universality and complexity of basic biochemical mechanisms, it would be surprising if some of the variety of polyP functions observed in microorganisms did not apply to aspects of human growth and development, to aging and to the aberrations of disease. Of theoretical interest regarding polyP is its antiquity in prebiotic evolution, which, along with its high energy and phosphate content, make it a plausible precursor to RNA, DNA and proteins. Of practical interest is its many industrial applications, among which is its use in the microbial depollution of Pi in marine environments.

Transcription of ppk from Acinetobacter sp. strain ADP1, encoding a putative polyphosphate kinase, is induced by phosphate starvation

Polyphosphate kinase (Ppk) catalyzes the formation of polyphosphate from ATP. We cloned the ppk gene (2,073 bp) from Acinetobacter sp. strain ADP1; this gene encodes a putative polypeptide of 78.6 kDa with extensive homology to polyphosphate kinase from Escherichia coli and other bacteria. Chromosomal disruption of ppk by inserting a transcriptionally fused lacZ does not affect growth under conditions of phosphate limitation or excess. beta-Galactosidase activity expressed from the single-copy ppk::lacZ fusion is induced 5- to 15-fold by phosphate starvation. An increased amount of ppk transcript (2.2 kb) was detected when cells were grown at a limiting phosphate concentration. Primer extension analysis revealed a regulated promoter located upstream of a second, constitutive promoter. Potential similarities of this regulation with the effects of PhoB and PhoR of E. coli are discussed.

Replacement of the active site tyrosine of vaccinia DNA topoisomerase by glutamate, cysteine or histidine converts the enzyme into a site-specific endonuclease

Vaccinia topoisomerase forms a covalent protein-DNA intermediate at 5'-CCCTT downward arrow sites in duplex DNA. The T downward arrow nucleotide is linked via a 3'-phosphodiester bond to Tyr-274 of the enzyme. Here, we report that mutant enzymes containing glutamate, cysteine or histidine in lieu of Tyr-274 catalyze endonucleolytic cleavage of a 60 bp duplex DNA at the CCCTT downward arrow site to yield a 3' phosphate-terminated product. The Cys-274 mutant forms trace levels of a covalent protein-DNA complex, suggesting that the DNA cleavage reaction may proceed through a cysteinyl-phosphate intermediate. However, the His-274 and Glu-274 mutants evince no detectable accumulation of a covalent protein-DNA adduct. Glu-274 is the most active of the mutants tested. The pH dependence of the endonuclease activity of Glu-274 (optimum pH = 6.5) is distinct from that of the wild-type enzyme in hydrolysis of the covalent adduct (optimum pH = 9.5). At pH 6.5, the Glu-274 endonuclease reaction is slower by 5-6 orders of magnitude than the rate of covalent adduct formation by the wild-type topoisomerase, but is approximately 20 times faster than the rate of hydrolysis by the wild-type covalent adduct. We discuss two potential mechanisms to account for the apparent conversion of a topoisomerase into an endonuclease.