Here, we report the first comprehensive study of Bartonella henselae gene expression during infection of human endothelial cells. Expression of the main cluster of upregulated genes, comprising the VirB type IV secretion system and its secreted protein substrates, is shown to be under the positive control of the transcriptional regulator BatR. We demonstrate binding of BatR to the promoters of the virB operon and a substrate-encoding gene and provide biochemical evidence that BatR and BatS constitute a functional twocomponent regulatory system. Moreover, in contrast to the acid-inducible (pH 5.5) homologs ChvG/ChvI of Agrobacterium tumefaciens, BatR/BatS are optimally activated at the physiological pH of blood (pH 7.4). By conservation analysis of the BatR regulon, we show that BatR/BatS are uniquely adapted to upregulate a genus-specific virulence regulon during hemotropic infection in mammals. Thus, we propose that BatR/BatS two-component system homologs represent vertically inherited pH sensors that control the expression of horizontally transmitted gene sets critical for the diverse host-associated life styles of the alphaproteobacteria.

Diazaborine treatment of yeast cells was shown previously to cause accumulation of aberrant, 3-elongated mRNAs. Here we demonstrate that the drug inhibits maturation of rRNAs for the large ribosomal subunit. Pulse-chase analyses showed that the processing of the 27S pre-rRNA to consecutive species was blocked in the drug-treated wild-type strain. The steady-state level of the 7S pre-rRNA was clearly reduced after short-term treatment with the inhibitor. At the same time an increase of the 35S pre-rRNA was observed. Longer incubation with the inhibitor resulted in a decrease of the 27S precursor. Primer extension assays showed that an early step in 27S pre-rRNA processing is inhibited, which results in an accumulation of the 27SA2 pre-rRNA and a strong decrease of the 27SA3, 27SB1L, and 27SB1S precursors. The rRNA processing pattern observed after diazaborine treatment resembles that reported after depletion of the RNA binding protein Nop4p/ Nop77p. This protein is essential for correct pre-27S rRNA processing. Using a green fluorescent protein-Nop4 fusion, we found that diazaborine treatment causes, within minutes, a rapid redistribution of the protein from the nucleolus to the periphery of the nucleus, which provides a possible explanation for the effect of diazaborine on rRNA processing.Translation of mRNA in the cytoplasm relies on the ribosome. While the principal steps in initiation and elongation of protein synthesis were uncovered in the middle of the last century, details of the biogenesis of the ribosome itself remained elusive. However, the development of novel techniques in affinity purification and refined techniques of protein identification by mass spectrometry within the last several years provided novel insights into the steps of assembly of the ribosomal subunits and how they are coordinated with the processing of the rRNA precursors (for recent reviews see references 15 and 47). Much of this detailed knowledge was worked out in the baker's yeast, Saccharomyces cerevisiae. This organism constitutes a simple model for the reaction sequence occurring in more complex eukaryotic cells.While the 5S rRNA is transcribed by RNA polymerase III, the 18S, 25S, and 5.8S rRNAs are transcribed as a single precursor molecule by RNA polymerase I. The primary transcript is cotranscriptionally processed at its 3Ј external transcribed spacer (ETS) end into the 35S pre-rRNA (26). The rRNAs for both the large and the small subunits are then generated from the 35S pre-rRNA by several subsequent processing events. In the first two processing steps for the 35S rRNA, the 5Ј ETS is successively removed by endonucleolytic cleavages at positions A0 and A1. The following step, which separates the rRNA precursors for the two ribosomal subunits, is a cleavage reaction at position A2 in internal transcribed spacer 1 (ITS1) located between the 18S and 5.8S rRNAs (23,27). This cleavage reaction results in the release of the 20S pre-rRNA, which matures in the cytoplasm to the 18S rRNA present in the small subunit of the ribosome ...