RESUMEN
In aerobically grown Azorhizobium caulinodans strain IRBG 46, in vivo expression of nitrate reductase (NR) and nitrite reductase (NiR) requires the presence of either nitrate or nitrite. On the contrary mere microaerobic conditions are sufficient for the expression of NR and NiR, however, addition of nitrate to the growth medium enhanced the activities of the enzymes. Optimum concentration of nitrate for maximum expression of NR and NiR activities was different in aerobic and microaerobic conditions. Nitrite was released into the medium both in aerobic and microaerobic conditions beyond a particular concentration of nitrate in the medium. Dissimilatory nitrate reduction was affected to a lesser extent by ammonium compared to assimilatory nitrate reduction.