Conservation of the links between gene transcription and chromosomal organization in the highly reduced genome of Buchnera aphidicola

Using an oligonucleotide-based microarray, we normalized the transcriptomic data by genomic DNA signals in order to have access to inter-gene comparison data. Our analysis showed that mRNA abundances, gene organization (operon) and gene essentiality are correlated in Buchnera (i.e., the most expressed genes are essential genes organized in operons) whereas no link between mRNA abundances and gene strand bias was found. The effect of Buchnera genome evolution on gene expression levels has also been analysed in order to assess the constraints imposed by the obligate symbiosis with aphids, underlining the importance of some gene sets for the survival of the two partners. Finally, our results show the existence of spatial periodic transcriptional patterns in the genome of Buchnera.Despite an important reduction in its genome size and an apparent decay of its capacity for regulating transcription, this work reveals a significant correlation between mRNA abundances and chromosomal organization of the aphid-symbiont Buchnera.Past genomic studies have comprehensively described the organization of the bacterial chromosome, for example in terms of gene localization, order and orientation. The degree of organization has been shown to increase with genome size, overall GC composition and the presence of nucleoid-binding proteins [1]. This organization of the chromosome can be described as being an adaptive and functional tool, essential for the survival of the bacterial cell. More precisely, several studies have identified strand asymmetries in the distribution of genes between the leading and the lagging strand of DNA (for a review see Rocha [2]). Indeed, bacterial genomes carry, on average, from 78% (for genomes containing the polymerase PolC) to 58% (for the other genomes) of their genes on the leading strand [3]. This bias is even more important when the essentiality of genes is taken into account, and essential genes distribution bias reaches 76% and 94% in Escherichia col