Replicon-Specific Regulation of Small Heat Shock Genes in Agrobacterium tumefaciens

Abstract

Four genes coding for small heat shock proteins (sHsps) were identified in the genome sequence of Agrobacterium tumefaciens, one on the circular chromosome (hspC), one on the linear chromosome (hspL), and two on the pAT plasmid (hspAT1 and hspAT2). Induction of sHsps at elevated temperatures was revealed by immunoblot analyses. Primer extension experiments and translational lacZ fusions demonstrated that expression of the pAT-derived genes and hspL is controlled by temperature in a regulon-specific manner. While the sHsp gene on the linear chromosome turned out to be regulated by RpoH ( 32 ), both copies on pAT were under the control of highly conserved ROSE (named for repression of heat shock gene expression) sequences in their 5 untranslated region. Secondary structure predictions of the corresponding mRNA strongly suggest that it represses translation at low temperatures by masking the Shine-Dalgarno sequence. The hspC gene was barely expressed (if at all) and not temperature responsive.The virulence of pathogenic microorganisms is strongly affected by temperature (11,16,41). Whereas a temperature of 37°C induces expression of virulence genes in mammalian pathogens, much lower temperatures are typically favorable for the infection process of plant pathogens. Many virulence genes in Agrobacterium tumefaciens, Pseudomonas syringae, and different Erwinia species are induced at temperatures between 18 and 28°C and repressed at higher temperatures (41). Various reasons account for the reduced infectivity of A. tumefaciens at temperatures above 28°C. (i) The signal transduction pathway mediated by the VirAG two-component regulatory system is functional only below this temperature (15). (ii) Assembly of T-pilus components required for DNA transfer to plant cells is compromised (3,4,8,9,17). (iii) Degradation of a subset of virulence proteins might occur at elevated temperatures (4).A puzzling observation was recently made during studies on the temperature stability of the A. tumefaciens type IV secretion system, which translocates DNA and protein into plant cells. Concomitant with the accumulation of plant secondary metabolite-induced virulence proteins at low temperatures, small heat shock proteins (sHsps) were induced (see Discussion of reference 4). Synthesis of sHsps was also observed during symbiosis of Sinorhizobium meliloti with the legume

Identification of the VirB4-VirB8-VirB5-VirB2 Pilus Assembly Sequence of Type IV Secretion Systems

“…N-terminally StrepII-tagged proteins were overproduced in GJ1158 using pT7-7StrepII (34), and 400-ml cultures were lysed in 10 ml of S2B (0.1 M Tris-HCl, pH 8, 0.15 M NaCl, 1 mM EDTA, pH 8) with 0.5 mM phenylmethylsulfonyl fluoride. The cells were lysed, and the supernatant was collected as described above.…”

TheEscherichia coliibpA thermometer is comprised of stable and unstable structural elements

“…13 Unstable base pairs are also the key to temperature sensing by the ROSE (Repression Of heat Shock gene Expression) element, which was discovered as a conserved sequence in the 5'-UTR of multiple small heat shock genes in rhizobia. [14][15][16] The RNA is predicted to fold into a complex structure consisting of three or four stem-loops. 10 In each case, the SD sequence is masked by imperfect base pairing in the partially conserved 3'-proximal stemloop.…”

Small heat shock proteins: recent developments

“…As described for Agrobacterium tumefaciens, it is also possible that several occurring sHSP genes are regulated differently in a replicon dependent manner. While the sHSP gene on the linear chromosome turned out to be regulated by RpoH, transcripts of the sHSP genes on the A. tumefaciens plasmids were under the control of ROSE sequences in their 5′ untranslated region (65). In summary, this tight and optimized regulation of the cytoplasmic stress response is important for unicellular organisms to cope with suddenly occurring stress and recovery conditions in an adequate way.…”