Chromosome organization and gene expression regulation in Buchnera aphidicola, the obligate intracellular bacteria of the pea aphid Acyrthosiphon pisum

Buchnera aphidicola, associated with the aphid scavengers (Hemiptera, Aphidoidea), is one of the most studied intracellular bacterium of insects. Even if we know that it can complement its host diet (especially with essential amino acids) and that it can adapt this furniture to the variation of the host demand, little is known about the mechanisms underlying this regulation. This work presents a systemic overview of the transcription regulation machinery in BuchneraAPS, associated with the pea aphid Acyrthosiphon pisum . The operon map of BuchneraAPS appears very compact including long recent transcription units with non-functionally related genes. A comparison with Escherichia coli reveals that the transcription units in the Buchnera lineage seem to inexorably increase in size due to mutations of the regulatory element at their borders. Despite, its small genome size, BuchneraAPS preserved long intergenic regions with conserved σ⁷⁰ and σ³² promoters. The high AT content of its genome favours both the flexibility and the curvature of the chromosome, and by the way, favours the fixation of the main Nucleoid Associated Proteins (NAP) highly conserved in the bacterium. Two sets of promoter regions (stable and unstable) were found in BuchneraAPS; regulator and transport genes were found associated with unstable regions. Gene expression is correlated with the physical parameters describing the DNA topology, and reveals a periodic pattern of 100kb along the chromosome of the bacterium. Hence, the regulatory network of BuchneraAPS seems mainly governed by several global toporegulators that might be able to sense the nutritional need of the aphid via the energetic level of the Buchnera cell (ie, the ATP/ADP ratio acting on gyrase activities influence the DNA helicity). Experimental work is needed in the future to validate these hypotheses.