We have studied the effect of assembly of ribosomal proteins S7, S9 and S19 on the accessibility and conformation of nucleotides in 16 S ribosomal RNA. Complexes formed between 16 S rRNA and S7, S7 + S9, S7 + S19 or S7 + S9 + S19 were subjected to a combination of chemical and enzymatic probes, whose sites of attack in 16 S rRNA were identified by primer extension. The results of this study show that: (1) Protein S7 affects the reactivity of an extensive region in the lower half of the 3' major domain. Inclusion of proteins S9 or S19 with S7 has generally little additional effect on S7-specific protection of the RNA. Clusters of nucleotides that are protected by protein S7 are localized in the 935-945 region, the 950/1230 stem, the 1250/1285 internal loop, and the 1350/1370 stem. (2) Addition of protein S9 in the presence of S7 causes several additional effects principally in two structurally distal regions. We observe strong S9-dependent protection of positions 1278 to 1283, and of several positions in the 1125/1145 internal loop. These findings suggest that interaction of protein S9 with 16 S rRNA results in a structure in which the 1125/1145 and 1280 regions are proximal to each other. (3) Most of the strong S19-dependent effects are clustered in the 950-1050 and 1210-1230 regions, which are joined by base-pairing in the 16 S rRNA secondary structure. The highly conserved 960-975 stemp-loop, which has been implicated in tRNA binding, appears to be destabilized in the presence of S19. (4) Protein S7 causes enhanced reactivity at several sites that become protected upon addition of S9 or S19. This suggests that S7-induced conformational changes in 16 S rRNA play a role in the co-operativity of assembly of the 3' major domain.