||Prokaryotes make extensive use of posttranscriptional regulation to modulate diverse cellular processes such as central carbon metabolism, stress response pathways, and virulence determinants. Posttranscriptional regulation in Escherichia coli is mediated via two broadly characterized methods. The first utilizes small noncoding RNAs (sRNAs) which bind target mRNA transcripts to alter their stability and translation. Nearly all characterized sRNAs function jointly with an RNA chaperone protein, Hfq. The second method employs mRNA-binding proteins which directly mediate translational inhibition or activation upon mRNA targets. Posttranscriptional regulation by both methods was recently demonstrated important to pathogenesis by several bacterial organisms. This study addresses the role of posttranscriptional regulation in uropathogenic Escherichia coli (UPEC), the organisms responsible for the majority of urinary tract infections. Specifically, deletion of Hfq, an RNA chaperone required for many sRNA-mRNA interactions, strongly reduced infection in murine models of cystitis and pyelonephritis and virtually eliminated formation of UPEC intracellular bacterial communities (IBCs). The hfq mutant experienced severe sensitivities to membrane disrupting agents such as polymyxin B, reactive oxygen species (ROS) and reactive nitrogen species (RNS) during in vitro models of host innate immune function. These phenotypes mirrored those of a !E-deleted UPEC, suggesting Hfq's involvement in posttranscriptional regulation of virulence was largely exerted at the bacterial envelope. In addition, RNS-treatment of ! ! "#! UPEC resulted in posttranscriptional downregulation of CpxP, a periplasmic regulator of the Cpx envelope stress response pathway. This downregulation was dependent on carbon storage regulator A (CsrA), a protein posttranscriptional regulator, as overexpression of CsrB, an sRNA antagonist of CsrA function, was sufficient to prevent as well as overcome downregulation of CpxP by RNS. Overexpression of CpxP in the presence of RNS proved beneficial to growth, however, suggesting CpxP downregulation by urinary RNS may not just disrupt UPEC's envelope, but impair the Cpx pathway involved in its repair. Anti-nitrotyrosine immunoblotting and mass-spectrometry indicate nitrosation of CsrA at tyrosine 48, a residue immediately adjacent to the domain implicated in RNA interaction, possibly altering CsrA's binding properties. These results demonstrate posttranscriptional regulation assisting virulence, but also imply manipulation by the host to deter growth.