MOLECULAR CHARACTERIZATION OF THE PRINCIPAL SYMBIOTIC BACTERIA OF THEWEEVIL SITOPHILUS-ORYZAE - A PECULIAR G+C CONTENT OF AN ENDOCYTOBIOTIC DNA

The principal intracellular symbiotic bacteria of the cereal weevil Si tophilus oryzae were characterized using the sequence of the 16S rDNA gene (rrs gene) and G + C content analysis. Polymerase chain reaction amplification with universal eubacterial primers of the rrs gene showe d a single expected sequence of 1,501 bp. Comparison of this sequence with the available database sequences placed the intracellular bacteri a of S. oryzae as members of the Enterobacteriaceae family, closely re lated to the free-living bacteria, Erwinia her and Escherichia coli, a nd the endocytobiotic bacteria of the tsetse fly and aphids. Moreover, by high-performance liquid chromatography, we measured the genomic G + C content of the S. oryzae principal endocytobiotes (SOPE) as 54%, w hile the known genomic G + C content of most intracellular bacteria is about 39.5%. Furthermore, based on the third codon position G + C con tent and the rrs gene G + C content, we demonstrated that most intrace llular bacteria except SOPE are A + T biased irrespective of their phy logenetic position. Finally, using the hsp60 gene sequence, the codon usage of SOPE was compared with that of two phylogenetically closely r elated bacteria: E. coli, a free-living bacterium, and Buchnera aphidi cola, the intracellular symbiotic bacteria of aphids. Taken together, these results show a peculiar and distinctly different DNA composition of SOPE with respect to the other obligate intracellular bacteria, an d, combined with biological and biochemical data, they elucidate the e volution of symbiosis in S. oryzae.