The endoribonuclease toxins of the E. coli toxin-antitoxin systems arrest bacterial growth and protein synthesis by targeting cellular mRNAs. As an exception, E. coli MazF was reported to cleave also 16S rRNA at a single site and separate an anti-Shine-Dalgarno sequence-containing RNA fragment from the ribosome. We noticed extensive rRNA fragmentation in response to induction of the toxins MazF and MqsR, which suggested that these toxins can cleave rRNA at multiple sites. We adapted differential RNA-sequencing to map the toxin-cleaved 5'- and 3'-ends. Our results show that the MazF and MqsR cleavage sites are located within structured rRNA regions and, therefore, are not accessible in assembled ribosomes. Most of the rRNA fragments are located in the aberrant ribosomal subunits that accumulate in response to toxin induction and contain unprocessed rRNA precursors. We did not detect MazF- or MqsR-cleaved rRNA in stationary phase bacteria and in assembled ribosomes. Thus, we conclude that MazF and MqsR cleave rRNA precursors before the ribosomes are assembled and potentially facilitate the decay of surplus rRNA transcripts during stress.

Uropathogenic strains of Escherichia coli (UPEC) are the major cause of bacteremic urinary tract infections. Survival in the bloodstream is associated with different mechanisms that help to resist serum complement-mediated killing. While the phenotypic heterogeneity of bacteria has been shown to influence antibiotic tolerance, the possibility that it makes cells refractory to killing by the immune system has not been experimentally tested. In the present study we sought to determine whether the heterogeneity of bacterial cultures is relevant to bacterial targeting by the serum complement system. We monitored cell divisions in the UPEC strain CFT073 with fluorescent reporter protein. T he immune system has several pathways for recognizing and killing pathogenic bacteria. However, some pathogenic bacteria can maintain infection in mammalian hosts despite inflammation, specific antimicrobial mechanisms, and a robust adaptive immune response and can therefore give rise to persistent infection (1). Uropathogenic Escherichia coli (UPEC) causes recurrent urinary tract infections that can progress from the lower to the upper urinary tract and can lead to the dissemination of bacteria into the bloodstream.The complement system is part of the defense against invading pathogens, with an essential role in both innate and adaptive immunity (2). It is composed of more than 40 plasma and membrane proteins. It can be activated via three distinct routes: the classical (antibody dependent), lectin, and alternative pathways. Activation of complement cascades leads to the formation of the key component C3b on the bacterial surface, which stimulates phagocytosis. Late complement components (C5 to C9 proteins) are also activated via C3b, resulting in the formation of the membrane attack complex (MAC) causing cell lysis (2). The primary source of complement is blood, but complement proteins are also synthesized by a variety of other cell types and tissues (3).Bacterial resistance to serum complement killing depends on the presence or absence of antigenic outer membrane proteins (4). Pathogens often resist recognition and subsequent complement activation owing to their surface capsular polysaccharide, which masks underlying structures and by itself activates complement poorly (5-7). In addition, secretion of the exopolysaccharide colanic acid protects UPEC from complement-mediated killing (8). Modification of lipopolysaccharide (LPS) is also important for complement evasion (9, 10). It has been shown that serum sensitivity depends on the bacterial growth phase; cells are more readily killed by serum during early logarithmic phase (11)(12)(13)(14). However, there are examples showing that exponential-phase cells are more resistant to complement-mediated killing than stationary-phase cells (15,16). That phenomenon is largely explained by the growth-phase-dependent expression of antigens, capsule, and LPS modification (14, 15). Cell size can also be an important determinant of complement-mediated killing, since larger or aggregated cells ...