The regulatory ACT domains serve as amino acid-binding sites in some amino acid metabolic enzymes and transcriptional regulators in bacteria. To elucidate the molecular roles of the glutamine (Gln)-sensing system in nitrogen (N) metabolism in plants, we isolated six genes encoding ACT domain repeat proteins (ACR1, and ACR5-ACR9) from rice (Oryza sativa L.) using genomic information on the primary structure composed of four copies of the domain homologous to those of bacterial Gln sensor GLND. Since expression of ACR7 was the most abundant of the six ACR orthologous genes, we focused on this ACR in the current study. Gene products of ACR7 were most abundant in young developing leaf blades of rice, and ACR7 protein is specifically localized in the nucleus of the parenchyma cells of phloem and xylem in the vascular bundles. A yeast two-hybrid screen identified a small heat stress protein (HSP18.0-CII) as a protein interacting with ACR7. Transient expression analysis of HSP18.0-CII:sGFP in cultured rice cells, followed by co-immunoprecipitation, suggests that the nuclear ACR7 indeed interacted with nucleocytoplasmic HSP18.0-CII in vivo. The potential ability of nuclear protein ACR7 to bind Gln and the possibility of the protein acting as a Gln sensor in rice leaves is discussed.