Bacterial cultures. Pure cultures of Nitrosomonas europaea (ATCC 25978 in ATCC medium 2265), Nitrosospira multiformis (ATCC 25196 in ATCC medium 929), and Nitrobacter agilis (ATCC 25384 in ATCC medium 480) were grown aerobically in 0.5-liter flasks at 30°C. The pH was kept at 8.0 by periodic addition of 20% Na2CO3. Cells were harvested by centrifugation at 3,200 g, and cell
pellets were processed for extraction of RNA.