MOLECULAR CELL BIOLOGICAL STUDIES ON INTRACELLULAR SYMBIOSIS

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We studied on Buchnera, intracellular symbiotic bacteria of aphids and intracellular yeast-like symbionts of brown planthoppers, the results of which are summarized as follows :1. Comparison of gene expression patterns of the bacteriocyte symbiotic systems from young and old aphids revealed that there are more than ten genes, of both host cell and Buchnera, that are specifically expressed only when the symbiotic system is kept functional. These genes include those for enzymes involved in the amino acid metabolism, confirming our previous results.2. We attained convincing evidence that Buchnera produce riboflavin and provision the vitamin to hosts.3. We estimated the genome size of Buchnera by digesting the genomic DNA with restriction enzymes whose restriction sites are rich in G/C, followed by pulse-field gel electrophoresis. It turned out that the genome size, 657kbp, is only one seventh of that of the genome of E.coli.4. Dot-blot analyses of two different genes and direct fluorimetry of the cellular DNA content revealed that Buchnera are very extraordinary bacteria with more than 100 genomic copies in a single cell. In addition, the genomic copy number of Buchnera is variable to a great extent due to morph and developmental stage of the host insect.5. Buchnera are exceptional as bacteria in that they contain a histone Hi -like protein, which is likely in interaction with the genomic DNA.6. We successfully cloned the gene for uricase of the yeast-like symbiont that is essential for planthoppers to mobilize the uric acid stored in their tissue. Also, we constructed a large scale expression system of the uricase gene by transforming E.coil with the gene ligated into an expression vector.