The aphid Schizaphis graminum is dependent on an association with a prokaryotic endosymbiont (Buchnera aphidicola). The nucleotide (nt) sequence of a 5040 base pair (bp) DNA fragment of B. aphidicola, homologous to the rplL-rpoB-rpoC portion of the Escherichia coli beta-operon, was determined. The DNA coded for the terminal 35 amino acids of Rp1L (large ribosomal subunit protein L7/L12), the complete RpoB (beta-subunit of RNA polymerase), and the first 209 amino acids of RpoC (beta'-subunit of RNA polymerase). The deduced sequences of B. aphidicola Rp1L, RpoB, and RpoC were 71, 84, and 91% identical, respectively, to the homologous proteins of E. coli. The sequences of two portions of the intergenic region between rp1L and rpoB were nearly identical in both B. aphidicola and E. coli. One sequence constituted an inverted repeat that could be an RNase III-messenger RNA processing site; the other sequence preceded RpoB. A compilation of the codon usage for RpoB, RpoC, and other B. apidicola proteins indicated a major preference for A or T in the first and third positions, a result consistent with the low guanine plus cytosine (G + C) content of the DNA of this organism.