The primary structure of the nucleic acid from the branching enzyme 1,4-1x-~-glucan : 1,4-a-Dglucan 6-a-( 1,4-a-gIucano)-transferase (2.5-S RNA) isolated from rabbit muscles has been elucidated. The polyribonucleotide consists of 31 nucleotides; the unique features of the polyribonucleotide are the unusually high content of modified nucleotides (32 %) and guanine residues (40 %). Apparently
2.5-S RNA belongs to a class of nucleic acids unknown up to now.It is the first time that the structure of a nucleic acid component from a ribonucleoenzyme has been defined. This work is a prerequisite for gaining insight into the intimate activating effect of the polyribonucleotide on the enzyme action.