Escherichia coli has a ribonucleoprotein complex that is composed of a 114 nucleotide 4.5S RNA and a 48 kDa polypeptide (P48) that has been demonstrated to function in translation and in the secretion of periplasmic polypeptides. A small RNA of approximately 220 nucleotides has been identified in maize mitochondria that includes sequence identity with the highly conserved domain of the bacterial 4.5S RNA. The transcript is mitochondrially encoded and maps to a region upstream of the gene for ATP synthase subunit I. The mitochondrial 4.5S-like RNA has 15 nucleotides of sequence identity with the highly conserved region of the bacterial 4.5S RNA. Sucrose density gradient centrifugation of a maize mitochondrial lysate demonstrated that the 4.5S RNA is a component of a high molecular weight complex under native conditions, and could be disrupted by phenol. Anti-P48 immune serum immuno-precipitated a mitochondrial protein of approximately 48 kDa, and RNA gel blot analysis of the immunoprecipitation reaction indicated that the 4.5S-like RNA co-immuno-precipitated with the 48 kDa polypeptide. The mitochondrial 4.5S ribonucleoprotein complex could function in translation or protein targeting.