Nitrate reductases in Escherichia coli

Area of Science:

• Microbiology
• Molecular Biology
• Enzymology

Background:

• Escherichia coli utilizes two distinct membrane-bound respiratory nitrate reductases: nitrate reductase A (NRA) and nitrate reductase Z (NRZ).
• These enzymes play crucial roles in bacterial respiration and nitrogen metabolism.

Purpose of the Study:

• To compare the genetic organization, biochemical characteristics, and regulatory mechanisms of NRA and NRZ in E. coli.
• To elucidate the differential expression patterns and potential physiological roles of these two nitrate reductases.

Main Methods:

• Comparative analysis of gene loci and operon structures (narGHJI for NRA, narZYWV for NRZ).
• Examination of biochemical properties and substrate utilization.
• Investigation of regulatory elements including transcription factors (Fnr, NarL) and two-component systems.

Main Results:

• NRA and NRZ share high sequence homology but are encoded by distinct operons at different chromosomal locations.
• NRA expression is tightly regulated by anaerobiosis (Fnr) and nitrate (NarL), while NRZ is constitutively expressed.
• NRA accounts for the vast majority (98%) of nitrate reductase activity upon induction.

Conclusions:

• Nitrate reductase A is the predominant respiratory nitrate reductase in E. coli.
• The specific physiological function of the constitutively expressed nitrate reductase Z requires further investigation.