ABSTRACT The chemolithotrophic ammonia-oxidizing bacterium Nitrosomonas europaea is known to be highly resistant to starvation conditions. The transcriptional response of N. europaea to ammonia addition following short- and long-term starvation was examined by primer extension and S1 nuclease protection analyses of genes encoding enzymes for ammonia oxidation ( amoCAB operons) and CO ₂ fixation ( cbbLS ), a third, lone copy of amoC ( amoC ₃ ), and two representative housekeeping genes ( glyA and rpsJ ). Primer extension analysis of RNA isolated from growing, starved, and recovering cells revealed two differentially regulated promoters upstream of the two amoCAB operons. The distal σ ⁷⁰ type amoCAB promoter was constitutively active in the presence of ammonia, but the proximal promoter was only active when cells were recovering from ammonia starvation. The lone, divergent copy of amoC ( amoC ₃ ) was expressed only during recovery. Both the proximal amoC _{1 , 2} promoter and the amoC ₃ promoter are similar to gram-negative σ ^E promoters, thus implicating σ ^E in the regulation of the recovery response. Although modeling of subunit interactions suggested that a nonconservative proline substitution in AmoC ₃ may modify the activity of the holoenzyme, characterization of a Δ amoC ₃ strain showed no significant difference in starvation recovery under conditions evaluated. In contrast to the amo transcripts, a delayed appearance of transcripts for a gene required for CO ₂ fixation ( cbbL ) suggested that its transcription is retarded until sufficient energy is available. Overall, these data revealed a programmed exit from starvation likely involving regulation by σ ^E and the coordinated regulation of catabolic and anabolic genes.