N2 fixers generally perform nitrogen fixation in low ammonia environments. However, our lab reveals that Paenibacillus spp. can perform nitrogen fixation under high (50-300 mM) concentration of ammonium. Paenibacillus sabinae T27 is representative which exerts nitrogen fixation under both nitrogen limitation and nitrogen excess conditions. Since the nitrogenase activity of P. sabinae T27 was significantly affected by addition of ammonium in the medium, we may ask how it happened in the transcriptome. In this study, microarray technology was adopted to answer this question.