Title : Different levels of transcriptional regulation to trophic constraints in the reduced 1 genome of Buchnera aphidicola APS 2

Abbreviation: EAA: essential amino acid; pTU: putative transcription unit 22 2 ABSTRACT 25 Symbiotic associations involving intracellular microorganisms and animals are 26 widespread, especially for species feeding on poor or unbalanced diets. Buchnera 27 aphidicola, the obligate intracellular bacteria associated with most aphid species, 28 provide their hosts with essential amino acids (EAAs), nutrients in short supply in the 29 plant phloem sap. The Buchnera genome has undergone severe reductions during 30 intracellular evolution. Genes for EAA biosynthesis are conserved but most of the 31 transcriptional regulatory elements are lost. This work addresses two main questions: 32 is transcription in Buchnera (i) regulated and (ii) scaled to aphid EAA demand? Two 33 microarray experiments were designed for profiling the gene expression in Buchnera. 34 The first one was characterized by a specific depletion of tyrosine and phenylalanine in 35 the aphid diet, and the second experiment combined a global diminution of EAAs in 36 the aphid diet with a sucrose concentration increase so as to manipulate the aphid 37 growth rate. Aphid biological performance and budget analysis (balance between 38 EAAs provided by the diet and synthesized by Buchnera) were performed to quantify 39 the nutritional demand from the aphids toward their symbiotic bacteria. Despite the 40 absence of known regulatory elements, a significant transcriptional regulation was 41 observed at different levels of organization in the Buchnera genome: between genes, 42 within putative transcription units and within specific metabolic pathways. However, 43 unambiguous evidence for transcriptional changes underpinning the scaling of EAA 44 biosynthesis to aphid demand was not obtained. The phenotypic relevance of the 45 transcriptional response from the reduced genome of Buchnera is addressed. 46