Transcriptional Processing of the pst Operon of Escherichia coli Meire Aguena Æ Beny Spira Received: 17 April 2008 / Accepted: 17 October 2008 / Published online: 19 November 2008 Ó Springer Science+Business Media, LLC 2008 Abstract The pst operon of Escherichia coli is composed of five genes that encode a high-affinity phosphate trans- port system. pst belongs to the PHO regulon, which is a group of genes and operons that are induced in response to phosphate limitation. The pst operon also has a regulatory role in the repression of PHO genes’ transcription under phosphate excess conditions. Transcription of pst is initi- ated at the promoter located upstream to the first gene, pstS. Immediately after its synthesis, the primary transcript of pst is cleaved into shorter mRNA molecules in a ribonuclease E-dependent manner. Other ribonucleases, such as RNase III and MazF, do not play a role in pst mRNA processing. RNase E is thus at least partially responsible for processing the pst primary transcript. Introduction The phosphate (Pi)-specific transport system of Esche- richia coli (Pst) is a typical ABC transport system composed of four different proteins: PstS, the periplasmic Pi-binding protein; PstC and PstA, integral membrane proteins that mediate the translocation of Pi through the inner membrane [24] and PstB that binds ATP and ener- gizes the transport [6]. The operon that encodes Pst contains five genes in the following order: pstS, pstC, pstA, pstB, and a fifth distal gene, phoU, whose product does not play a role in the transport of Pi [21]. The operon is located at minute 83 in the chromosome of E. coli and is tran- scribed counterclockwise [15]. As a member of the PHO regulon, the pst operon is induced in response to Pi limi- tation. All PHO genes are regulated by a two-component system formed by the proteins PhoB and PhoR. PhoR is a membranal sensor kinase that autophosphorylates in response to external low-Pi concentration and phosphory- lates PhoB [23]. Phosphorylated PhoB binds to the PHO boxes on the promoter site and interacts with RNA poly- merase, which initiates transcription [12]. The Pst system and PhoU also act as repressors of PHO transcription in cells grown under Pi excess, but the mechanism of this repression is still unclear [23]. Transcription of the five pst genes is initiated at the promoter upstream of the first gene, pstS. This is a typical PHO promoter with a -10 region and two PHO boxes. Because of its instability, the full-length transcript of pst could be detected only with an enhanced reverse tran- scription-polymerase chain reaction (RT-PCR) method [1, 2], whereas Northern hybridizations revealed small pst transcripts ranging from 0.9 to 2.6 Kb [2]. Here, we report that the transcript of pst is processed by the endoribonuc- lease RNase E and that the full-length transcript of pst could be observed by Northern blot analysis in a RNase E conditional mutant. We also show that other ribonucleases, such as RNase III or MazF, do not play a role in pst mRNA processing. Materials and Methods Bacterial Strains The strains used in this study were MG1655 (wild-type E. coli K-12 [25]), DmazEF (MC4100 DmazEF [3]), SK5665 M. Aguena Á B. Spira (&) Departamento de Microbiologia, Instituto de Cie ˆncias Biome ´dicas, Universidade de Sa ˜o Paulo, Av. Prof. Lineu Prestes, 1374, Sa ˜o Paulo, SP CEP 05508-900, Brazil e-mail: benys@usp.br 123 Curr Microbiol (2009) 58:264–267 DOI 10.1007/s00284-008-9319-1