Stability of the pstS transcript of Escherichia coli
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ORIGINAL PAPER
Stability of the pstS transcript of Escherichia coli Meire Aguena · Gerson Moura Ferreira · Beny Spira
Received: 30 April 2008 / Revised: 8 July 2008 / Accepted: 10 September 2008 / Published online: 27 September 2008 © Springer-Verlag 2008
Abstract The pst operon of Escherichia coli is composed of Wve genes that encode a high-aYnity phosphate transport system. As a member of the PHO regulon, pst transcription is activated under phosphate shortage conditions. Under phosphate-replete conditions, the pst operon also functions as a negative regulator of the PHO genes. Transcription of pst is initiated at the promoter located upstream to the Wrst gene, pstS. Immediately after its synthesis, the primary transcript of pst is cleaved into shorter mRNA molecules. The transcription unit corresponding to pstS is signiWcantly more abundant than the transcripts of the other pst genes due to stabilisation of pstS mRNA by a repetitive extragenic palindrome (REP) structure downstream to the pstS locus. The presence of the REP sequence also results in an increased level of PstS proteins. However, the surplus level of PstS proteins produced in the presence of REP does not contribute to the repressive role of Pst in PHO expression. Keywords
pst operon · PHO regulon · mRNA half-life
Introduction Escherichia coli possesses two transport systems for orthophosphate (Pi), Pit and Pst. Pit is a low aYnity and highvelocity Pi-transporter that consists of a single membranal
Communicated by Jorge Membrillo-Hernández. M. Aguena · G. M. Ferreira · B. Spira (&) Departamento de Microbiologia, Instituto de Ciências Biomédicas, Universidade de São Paulo, Av. Prof. Lineu Prestes, 1374, São Paulo, SP CEP 05508-900, Brazil e-mail: [email protected]
protein. It is encoded by the gene pit and it is synthesised constitutively. Pst is a typical ABC transport system composed of four diVerent proteins: PstS, the periplasmic Pibinding protein; PstC and PstA, integral membrane proteins that mediate the translocation of Pi through the inner membrane (Webb et al. 1992) and PstB that binds ATP and energises the transport (Chan and Torriani 1996). Pst is encoded by an operon of the same name (Fig. 1a), which besides the four genes encoding the transport system- pstS, pstC, pstA and pstB, contains a Wfth distal gene- phoU, whose product does not play a role in Pi transport (Steed and Wanner 1993). The pst operon belongs to the PHO regulon and is induced in response to Pi-limitation. Transcription of the PHO genes is regulated by the two- component system PhoB/PhoR. PhoR is a membranal sensor kinase that autophosphorylates in response to external low-Pi concentration and phosphorylates PhoB (Wanner 1996). Phosphorylated PhoB binds to consensus sequences in the promoter regions of all PHO genes, the PHO-boxes, where it interacts with RNA polymerase associated with 70 and initiates transcription of the PHO genes (Makino et al. 1993). Most mutations in any gene of the pst operon result in the constitutive expression of the PHO regulon even under Pi-ex
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