We determined that ribosomes of seedling roots of maize (Zea mays 1.) contain the acidic phosphoproteins (P-proteins) known to form a flexible lateral stalk structure of the 60s subunit of eukaryotic ribosomes. l h e P-protein stalk, composed of PO, P1, and P2, interacts with elongation factors, mRNA, and tRNA during translation. Acidic proteins of 13 to 15.5 k D were released as a complex from ribosomes with 0.4 M NH,CI/SO% ethanol. Protein and cDNA sequence analysis confirmed that maize ribosomes contain one type of P1, two types of P2, and a fourth and novel Pl/PZ-type protein.This novel P-protein, designated P3, has the conserved C terminus of P1 and P2. P1, P2, and P3 are similar in deduced m a s (1 1.4-1 2.2 kD) and isoelectric point (4.1-4.3). A 35.5-t o 36-kD acidic protein was released at low levels from ribosomes with 1 .O M NH,C1/50% ethanol and identified as PO. Labeling of roots with [32P]inorganic phosphate confirmed the in vivo phosphorylation of the P-proteins.Flooding caused dynamic changes in the P-protein complex, which affected the potential of ribosome-associated kinases and casein kinase II t o phosphorylate the P-proteins. We discuss possible alterations of the ribosomal P-protein complex and consider that these changes may be involved in the selective translation of mRNA in flooded roots.Ribosomes are a two-subunit organelle, and are the site of mRNA translation into protein in a11 organisms. The large ribosomal subunit is a complex macromolecule that is composed of rRNAs, a large number of basic (high-pI) proteins, and a small number of acidic (low-pI) proteins. Across evolutionary kingdoms and phyla the large ribosoma1 subunit is variable in size, but possesses a number of morphological features that are universally conserved. For example, a universal feature of the peptidyl transferase region of the large subunit is a complex of acidic proteins that form the body and stalk of a lateral protuberance (Moller, 1990; Liljas, 1991).In bacteria the acidic protein stalk of the 50s ribosomal subunit is composed of ribosomal protein, L10, and two dimers of L7 and L12 in a (L7/L12),-LlO pentameric complex (Moller, 1990; Liljas, 1991). L10 is a 17-kD acidic pro- tein that interacts with the 23s rRNA scaffold of the large subunit within the GTPase domain of the rRNA. L7 and L12 are 12-kD acidic proteins that are encoded by a single gene, but differ in that the N-terminal Ser of L7 is posttranslationally aminoacetylated. The N-terminal domain of L7 and L12 forms an a-helical structure that is responsible for dimer formation and binding to M O . A central region of acidic residues forms a flexible hinge that allows the dimers to assume an elongated conformation that forms the stalk or a closed conformation in which the C-and N-terminal domains are in close proximity on the body of the 50s ribosomal subunit (Oleinikov et al., 1993;Traut et al., 1993). The C-terminal regions of L7 and L12 are required for binding of elongation factor G, and the subsequent hydrolysis of GTP that occurs in the transloca...