BUCHNERA APHIDICOLA (A PROKARYOTIC ENDOSYMBIONT OF APHIDS) CONTAINS APUTATIVE 16S RIBOSOMAL-RNA OPERON UNLINKED TO THE 23S RIBOSOMAL-RNA-ENCODING GENE - SEQUENCE DETERMINATION, AND PROMOTER AND TERMINATOR ANALYSIS

The aphid Schizaphis graminum is dependent on an association with Buch nera aphidicola, an eubacterial endosymbiont located in specialized ho st cells. Past studies have indicated that Escherichia coli is the clo sest known relative of the endosymbiont which has many genetic attribu tes of free-living bacteria. In order to obtain information on the pro perties of highly expressed genes, we have chosen for study the single -copy rrs (gene encoding 16S rRNA) of B. aphidicola. A 4.4-kb DNA frag ment was cloned into E. coli and the nucleotide (nt) sequence determin ed. Several ORFs were identified; the order of genes was argS-rss-ORF1 -rnh-dnaQ. ArgS, RNase H and DnaQ had 36-57% amino acid (aa) identity to the homologous proteins of E. coli. B. aphidicola rrs appears to be part of an operon consisting of a putative promoter, rrs and two inve rted repeats resembling Rho-independent terminators. Comparisons of th e sequences of argS-rrn DNA fragments from endosymbionts of six additi onal aphid species indicated conservation of sequences corresponding t o a single - 35 (TTGACA) and - 10 (TGTAAT) promoter region, as well as boxA (sequence involved in antitermination) and boxC. The B. aphidico la argS-rrn DNA fragments from endosymbionts from seven species of aph ids had promoter activities in E. coli which ranged from 6 tb 135% of that observed with a comparable DNA fragment of E. coli rrnB. Similarl y, the putative B. aphidicola terminator was functional in E. coli. In most eubacteria, the rRNA-encoding genes are arranged in the order, 1 6S, 23S, 5S, and are part of a single operon. B. aphidicola differs fr om these organisms in that the gene encoding 16S rRNA. is on a transcr iption unit distinct from that for 23S and 5S rRNA.