Functional 30S ribosomes were reconstructed from total Escherichia coli 30S ribosomal proteins and 16S ribosomal RNA synthesized in vitro by T7 RNA polymerase. Up to 700 mol of RNA/mol of template could be obtained. The transcript lacked all ten normally modified bases and had three additional 5' G residues, an A----G change at position 2, and, in 22% of the molecules, one or two extra 3' residues. The synthetic 16S RNA could be assembled into a particle that cosedimented with authentic 30S and was indistinguishable from 30S by electron microscopy. When supplemented with the 50S subunit, the particles bound tRNA to the 70S P site in a codon- and Mg2+-dependent manner. The specific binding activity was 94% that of particles reconstituted with natural rRNA and 52% that of native 30S. Cross-linking to P site bound tRNA was also preserved. Changing C-1400, the residue known to be close to the anticodon of P site bound tRNA, to A had little effect on reconstitution, but the C----G substitution caused a marked inhibition of assembly. tRNA could bind to both reconstituted mutants, but cross-linking was greatly reduced. These results show that none of the modified bases of 16S RNA are essential for P site binding and that position 1400 may be more important for ribosome assembly than for tRNA binding. Base-specific in vitro mutagenesis can now be used to explore in detail the functional properties of individual residues in ribosomal RNA.