Fig. 1. Distinct transcriptional profile of the Y. pestis ΔphoP mutant in the flea. (a) Principal components analysis of five independent replicate transcriptional profiles of Y. pestis WT and isogenic ΔphoP strains during flea gut infection. (b) Venn diagrams representing the number of genes significantly upregulated (left) or downregulated (right) in WT Y. pestis relative to the ΔphoP mutant under different environmental conditions.