Detecting chemical signals is important for identifying food sources and avoiding harmful agents. Like most animals, use olfaction to chemotax towards their main food source, bacteria. However, little is known about the bacterial compounds governing attraction to bacteria and the physiological importance of these compounds to bacteria. Here, we address these questions by investigating the function of a small RNA, P11, in the pathogen, that was previously shown to mediate learned pathogen avoidance. We discovered that this RNA also affects the attraction of untrained to and does so by controlling production of ammonia, a volatile odorant produced during nitrogen assimilation. We untangle the complex regulation of nitrogen assimilation, which is mediated by a partner-switching mechanism involving environmental nitrates, sensor proteins, and P11. In addition to mediating attraction, nitrogen assimilation is important for bacterial fitness and pathogenesis during infection by . These studies define ammonia as a major mediator of trans-kingdom signaling, reveal the physiological importance of nitrogen assimilation for both bacteria and host organisms, and highlight how a bacterial metabolic pathway can either benefit or harm a host in different contexts.