Le génome d'une bactérie, Carsonella, vient d'être séquencé. C'est de loin le plus petit : 160 kb avec un GC% de 16 seulement.
La surprise : il lui manque des gènes essentiels, et les auteurs pensent qu'elle est en train de perdre ses gènes au profit de son hôte devenant ainsi un endosymbionte comme chloroplastes et mitochondries (peut-être aussi peroxysomes).
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Envoyé par Science mag Carsonella ruddii is a bacteriocyte-associated {gamma}-proteobacterial symbiont that appears to be present in all species of phloem sap-feeding insects, psyllids |
Le génome est très dense avec des cadres de lecture chevauchants, comme dans les virus.
Citation:
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Envoyé par Science mag Another feature of this genome is an extremely high gene density. The protein-coding sequences and RNA genes (one 16S-23S-5S ribosomal RNA operon and 28 tRNA genes for all 20 amino acids) cover 97.3% of the genome, which is a gene density higher than those in known bacterial genomes. This density is attributable to numerous overlapping genes. Of 182 ORFs, 164 (90%) overlap with at least one of the two adjacent ORFs, and the average length of all 132 overlaps is 10.7 bases. The majority (92%) are tandem overlaps on the same strand, all of which are out of frame. Moreover, the average length of Carsonella ORFs (826 bp) is notably shorter than that of other bacteria. Indeed, a comparison of 89 orthologous ORFs conserved in Carsonella and in seven bacteriocyte-restricted endosymbionts revealed that the average length of the ORFs in Carsonella is 17.8 to 18.4% shorter than the average ORF lengths of the other endosymbionts. |
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