Transcription regulation of the colicin K cka gene reveals induction of colicin synthesis by differential responses to environmental signals

Colicin-producing strains occur frequently in natural populations of Escherichia coli, and colicinogenicity seems to provide a competitive advantage in the natural habitat. A cka-lacZ fusion was used to study the regulation of expression of the colicin K structural gene. Expression is growth phase dependent, with high activity in the late stationary phase. Nutrient depletion induces the expression of cka due to an increase in ppGpp. Temperature is a strong signal for cka expression, since only basal-level activity was detected at 22 degrees C. Mitomycin C induction demonstrates that cka expression is regulated to a lesser extent by the SOS response independently of ppGpp. Increased osmolarity induces a partial increase, while the global regulator integration host factor inhibits expression in the late stationary phase. Induction of cka was demonstrated to be independent of the cyclic AMP-Crp complex, carbon source, RpoS, Lrp, H-NS, pH, and short-chain fatty acids. In contrast to colicin E1, cka expression is independent of catabolite repression and is partially affected by anaerobiosis only upon SOS induction. These results indicate that while different colicins are expressed in response to some common signals such as nutrient depletion, the expression of individual colicins could be further influenced by specific environmental cues.     

Regulation of nitrogen starvation responses by the alarmone (p)ppGpp in rice

Nitrogen is an essential macronutrient for all living organisms and is critical for crop productivity and quality.In higher plants, inorganic nitrogen is absorbed through roots and then assimilated into amino acids by the highly conserved glutamine synthetase/glutamine:2-oxoglutarate aminotransferase(GS/GOGAT) cycle.How nitrogen metabolism and nitrogen starvation responses of plants are regulated remains largely unknown. Previous studies revealed that mutations in the rice ABNORMAL CYTOKININ RES...     

The role of stress in colicin regulation

Bacteriocins produced by Enterobacteriaceae are high molecular weight toxic proteins that kill target cells through a variety of mechanisms, including pore formation and nucleic acid degradation. What is remarkable about these toxins is that their expression results in death to the producing cells and therefore bacteriocin induction have to be tightly regulated, often confined to times of stress. Information on the regulation of bacteriocins produced by enteric bacteria is sketchy as their expression has only been elucidated in a handful of bacteria. Here, we review the known regulatory mechanisms of enteric bacteriocins and explore the expression of 12 of them in response to various triggers: DNA-damaging agents, stringent response, catabolite repression, oxidative stress, growth phase, osmolarity, cold shock, nutrient deprivation, anaerobiosis and pH stress. Our results indicate that the expression of bacteriocins is mostly confined to mutagenic triggers, while all other triggers tested are limited inducers.     

Effective symbiosis between Rhizobium etli and Phaseolus vulgaris requires the alarmone ppGpp

The symbiotic interaction between Rhizobium etli and Phaseolus vulgaris, the common bean plant, ultimately results in the formation of nitrogen-fixing nodules. Many aspects of the intermediate and late stages of this interaction are still poorly understood. The R. etli relA gene was identified through a genome-wide screening for R. etli symbiotic mutants. RelA has a pivotal role in cellular physiology, as it catalyzes the synthesis of (p)ppGpp, which mediates the stringent response in bacteria. The synthesis of ppGpp was abolished in an R. etli relA mutant strain under conditions of amino acid starvation. Plants nodulated by an R. etli relA mutant had a strongly reduced nitrogen fixation activity (75% reduction). Also, at the microscopic level, bacteroid morphology was altered, with the size of relA mutant bacteroids being increased compared to that of wild-type bacteroids. The expression of the sigma(N)-dependent nitrogen fixation genes rpoN2 and iscN was considerably reduced in the relA mutant. In addition, the expression of the relA gene was negatively regulated by RpoN2, the symbiosis-specific sigma(N) copy of R. etli. Therefore, an autoregulatory loop controlling the expression of relA and rpoN2 seems operative in bacteroids. The production of long- and short-chain acyl-homoserine-lactones by the cinIR and raiIR systems was decreased in an R. etli relA mutant. Our results suggest that relA may play an important role in the regulation of gene expression in R. etli bacteroids and in the adaptation of bacteroid physiology.     

Structure of the PPX/GPPA phosphatase from Aquifex aeolicus in complex with the alarmone ppGpp

The crystal structure of the prototype exopolyphosphatase/guanosine pentaphosphate phosphohydrolase protein family member from Aquifex aeolicus in complex with the intracellular second messenger guanosine tetraphosphate was determined at 2.7-Å resolution. The hydrolytic base is identified as E119. The dual specificity established for the Escherichia coli homolog is shown to be compatible with a common active site for guanosine pentaphosphate and polyphosphate hydrolysis. Distinct and different degrees of closure between the two domains of the enzyme are associated with substrate binding. The arginines R22 and R267, residing in different domains, are crucial for guanosine pentaphosphate specificity as they interact with the unique 3′-ribose phosphorylation.     

Inhibition of colicin synthesis by the antibiotic globomycin

Wolinella succinogenes can grow by anaerobic respiration with fumarate or polysulfide as the terminal electron acceptor, and H₂ or formate as the electron donor. A ΔhydABC mutant lacking the hydrogenase structural genes did not grow with H₂ and either fumarate or polysulfide. In contrast to the wild-type strain, the mutant grown with fumarate and with formate instead of H₂ did not catalyze the reduction of fumarate, polysulfide, dimethylnaphthoquinone, or benzyl viologen by H₂. Growth and enzymic activities were restored upon integration of a plasmid carrying hydABC into the genome of the ΔhydABC mutant. The ΔhydABC mutant was complemented with hydABC operons modified by artificial stop codons in hydA (StopA) or at the 5′-end of hydC (StopC). The StopC mutant lacked HydC, and the hydrophobic C-terminus of HydA was missing in the hydrogenase of the StopA mutant. The two mutants catalyzed benzyl viologen reduction by H₂. The enzyme activity was located in the membrane of the mutants. A mutant with both modifications (StopAC) contained the activity in the periplasm. The three mutants did not grow with H₂ and either fumarate or polysulfide, and did not catalyze dimethylnaphthoquinone reduction by H₂. We conclude that the same hydrogenase serves in the anaerobic respiration with fumarate and with polysulfide. HydC and the C-terminus of HydA appear to be required for both routes of electron transport and for dimethylnaphthoquinone reduction by H₂. The hydrogenase is anchored in the membrane by HydC and by the C-terminus of HydA. The catalytic subunit HydB is oriented towards the periplasmic side of the membrane. Received: 29 December 1997 / Accepted: 6 March 1998