Role of the Escherichia coli nitrate transport protein, NarU, in survival during severe nutrient starvation and slow growth

Abstract

Escherichia coli K-12 strains expressing either NarU or NarK as the only nitrate transport protein are both able to support nitrate-dependent anaerobic growth. The narK gene is highly expressed during anaerobic growth in the presence of nitrate, consistent with a role for NarK in nitrate transport coupled to nitrate reduction by the most active nitrate reductase encoded by the adjacent narGHJI operon. The physiological role of NarU is unknown. Reverse transcriptase PCR experiments established that, unlike the monocistronic narK gene, narU is co-transcribed with narZ as the first gene of a five-gene narUZYWV operon. The narK and narU genes were fused in-frame to a myc tag: the encoded fusion proteins complemented the nitrate-dependent growth defect of chromosomal narK and narU mutations. A commercial anti-Myc antibody was used to detect NarK and NarU in membrane fractions. During anaerobic growth in the presence of nitrate, the quantity of NarU-Myc accumulated during exponential growth was far less than that of NarK-Myc, but NarU was more abundant than NarK in stationary-phase cultures in the absence of nitrate. Although the concentration of NarU-Myc increased considerably during the post-exponential phase of growth, NarK-Myc was still more abundant than NarU-Myc in stationary-phase bacteria in the presence of nitrate. In chemostat competition experiments, a strain expressing only narU had a selective advantage relative to a strain expressing only narK during nutrient starvation or very slow growth, but NarK + bacteria had a much greater selective advantage during rapid growth. The data suggest that NarU confers a selective advantage during severe nutrient starvation or slow growth, conditions similar to those encountered in vivo.

Resistance and survival strategies of Salmonella enterica to environmental stresses

“…In agreement with its complex regulation by known stress regulators, NR-Z is essential for long-term C-starvation survival, CSI cross-resistance to high temperature and acid pH as well as H 2 O 2 -inducible adaptive H 2 O 2 resistance (Seymour et al, 1996;Spector & Cubitt, 1992;. Clegg et al (2006) showed that in E. coli NarU accumulates in C-starved and chemostat-slow growing cells (with or without nitrate) and provides a selective advantage in the absence of the nitrate-inducible anaerobiosisinducible NarK nitrate transporter to cells during slow-growth and starvation conditions. However, their studies were carried out in the presence of (albeit limiting) nitrate concentrations.…”

“…Nonetheless, the importance of alternative respiratory systems in (aerobically grown) C-starved cells is supported by the finding that previously described stiA mutations are in the narZ gene. The narZ gene is the second gene of the narUZYWV operon encoding a nitrite extruder/nitrate transporter protein (NarU) and a second, so-called cryptic, nitrate reductase NR-Z (Clegg, Jiam, & Cole, 2006;. NR-Z is nitrate-unresponsive and anaerobiosis-repressed.…”

Global transcriptional response to vancomycin in Mycobacterium tuberculosis

“…The proteins encoded by these genes use and regenerate reducing equivalents from fatty acids in the absence of external electron acceptors (icl, pdhA and scoA) (Boshoff & Barry, 2005), or during the microaerophilic respiration that is typical of cultures subjected to severe nutrient starvation or slow growth (ndh, narU) (Boshoff & Barry, 2005;Clegg et al, 2006). Cell wall and cell processes.…”

The NapF protein of the Escherichia coli periplasmic nitrate reductase system: demonstration of a cytoplasmic location and interaction with the catalytic subunit, NapA

“…The membrane-bound nitrate reductases NarA and NarZ reduce nitrate in the cytoplasm (Berks et al, 1995a;Blasco et al, 1990;Iobbi-Nivol et al, 1990;Stewart, 1988). Nitrate reductase A is the predominant nitrate reductase when E. coli is grown under nitrate-sufficient conditions , whereas the poorly expressed nitrate reductase Z confers a selective advantage during the stationary phase or very slow growth (Iobbi et al, 1987Clegg et al, 2006). Under nitrate-limiting conditions, a periplasmic nitrate reductase (Nap) is expressed Wang et al, 1999;Stewart et al, 2002).…”