Protein HD40 , an RNA-helix destabilizing protein (Mr 40 000) is the major component of 30S heterogeneous nuclear ribonucleoprotein particles (hnRNP) from Artemia salina. The physical properties and the amino acid composition of HD40 are analogous to those of a group of well conserved core hnRNP proteins of higher eukaryotes. HD40 binds to and disrupts the secondary structure of single-stranded polynucleotides and forms extended nucleoprotein filaments at a stoichiometry of one protein per 12-15 nucleotides. The addition of a saturating amount of HD40 (one protein per 8 nucleotides) converts the filaments into bead-like complexes that are similar in properties and appearance to native hnRNP. To gain an understanding of the structure of hnRNP, we have used analytical ultracentrifugation and electron microscopy to investigate the structure of beaded complexes reconstituted from HD40 and poly(A)n of defined sizes. A complex containing 160 nucleotides forms a disc that is 3 nm high by 18 nm in diameter. At n less than 160 the complexes form sectors of the disc: 40 nucleotides give rise to a quarter of a disc, 80 nucleotides, half a disc, etc. At n greater than 160, the additional nucleoprotein elements may either initiate the formation of a second disc adjacent to the first or stack on top of the first disc to form a 6 nm high helix with a diameter of 18 nm. A single "bead" sediments at approximately 30S and contains an average of approximately 300 nucleotides and 1.8 turns of the helix. Native hnRNP particles from A. salina sediment at about 30S, have a diameter of 18-20 nm, and contain RNA fragments 180 to 400 nucleotides long.