Abstract

Buchnera aphidicola is a Gram⁻ eubacterium with a DNA G+C content of 28–30 mol%. This organism is an obligate intracellular symbiont of aphids. To determine its similarity to or difference from other eubacteria, a 4.9-kb DNA fragment from B. aphidicola containing the gene homologous to Escherichia coli dnaA (a gene involved in the initiation of chromosome replication) was cloned into E. coli and sequenced. The order of genes on this fragment, 60K-10K-rnpA-rpmH-dnaA-dnaN-gyrB. was similar to that found in other eubacteria. The sole difference was the absence of recF between dnaN and gyrB. The deduced amino acid sequence of these proteins resembled those of E. coli by a 41 to 83% identity. Except for E. coli, in all the eubacteria so far examined, dnaA is preceded by multiple 9-nucleotide repeats known as a DnaA boxes. No DnaA boxes were detected in the endosymbiont DNA. The possibility that this observation is a consequence of the low G+C content of this DNA fragment (14 mol% G+C) is unlikely since in Mycoplasma capricolum this fragment (19 mol% G+C) has eight DnaA boxes (Fujita et al., 1992). The presence of the sequence, GATC, recognized by the Dam methyltransferase system, only within six regions coding for proteins suggests that methylation is not a factor in the regulation of the initiation of endosymbiont chromosome replication.

Author Keywords: Buchnera aphidicola; Schizaphis graminum; molecular evolution; ribonuclease P; DnaA box; ribosomal protein L34; DNA polymerase III; gyrase subunit β

Abbreviations: aa, amino acid(s); B., Buchnera; bp, base pair(s); G+C, guanine+cytosine; kb, kilobase(s) or 1000 bp; nt, nucleotide(s); oligo, oligodeoxyribonucleotide; ORF, open reading frame; ori, origin of DNA replication; PCR, polymerase chain reaction; RBS, ribosome-binding site(s); RnpA, ribonuclease P protein; RpmH, large ribosomal subunit protein 34; 10K, ORFs coding for 10-; 60K, ORFs coding for 60-

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Corresponding author. Correspondence to: Dr. P. Baumann, Department of Microbiology, University of California, , Davis, CA 95616-8665 , U.S.A. Tel. (916)752-0272 or 752-1088; Fax (916)752-9014.