Locus of enterocyte effacement-encoded regulator
The locus of enterocyte effacement-encoded regulator (Ler) is a regulatory protein that controls bacterial pathogenicity of enteropathogenic Escherichia coli (EPEC) and enterohemorrhagic Escherichia coli (EHEC).{{Cite journal|last1=Mellies|first1=J. L.|last2=Elliott|first2=S. J.|last3=Sperandio|first3=V.|last4=Donnenberg|first4=M. S.|last5=Kaper|first5=J. B.|date=July 1999|title=The Per regulon of enteropathogenic Escherichia coli : identification of a regulatory cascade and a novel transcriptional activator, the locus of enterocyte effacement (LEE)-encoded regulator (Ler)|journal=Molecular Microbiology|volume=33|issue=2|pages=296–306|issn=0950-382X|pmid=10411746|doi=10.1046/j.1365-2958.1999.01473.x|s2cid=23881901}} More specifically, Ler regulates the locus of enterocyte effacement (LEE) pathogenicity island genes, which are responsible for creating intestinal attachment and effacing lesions and subsequent diarrhea: LEE1, LEE2, and LEE3. LEE1, 2, and 3 carry the information necessary for a type III secretion system. The transcript encoding the Ler protein is the open reading frame 1 on the LEE1 operon.
The mechanism of Ler regulation involves competition with histone-like nucleoid structuring protein (H-NS), a negative regulator of the LEE pathogenicity island.{{Cite journal|last1=Bustamante|first1=V. H.|last2=Santana|first2=F. J.|last3=Calva|first3=E.|last4=Puente|first4=J. L.|date=February 2001|title=Transcriptional regulation of type III secretion genes in enteropathogenic Escherichia coli: Ler antagonizes H-NS-dependent repression|journal=Molecular Microbiology|volume=39|issue=3|pages=664–678|issn=0950-382X|pmid=11169107|doi=10.1046/j.1365-2958.2001.02209.x|doi-access=free}} Ler is regulated by many factors such as plasmid encoded regulator (Per), integration host factor, Fis, BipA, a positive regulatory loop involving GrlA, and quorum sensing mediated by luxS.{{Cite journal|last1=Sircili|first1=Marcelo P.|last2=Walters|first2=Matthew|last3=Trabulsi|first3=Luis R.|last4=Sperandio|first4=Vanessa|date=2004-04-01|title=Modulation of Enteropathogenic Escherichia coli Virulence by Quorum Sensing|url= |journal=Infection and Immunity|language=en|volume=72|issue=4|pages=2329–2337|doi=10.1128/iai.72.4.2329-2337.2004|issn=0019-9567|pmid=15039358|pmc=375187}}{{Cite journal|last1=Barba|first1=Jeannette|last2=Bustamante|first2=Víctor H.|last3=Flores-Valdez|first3=Mario A.|last4=Deng|first4=Wanyin|last5=Finlay|first5=B. Brett|last6=Puente|first6=José L.|date=2005-12-01|title=A Positive Regulatory Loop Controls Expression of the Locus of Enterocyte Effacement-Encoded Regulators Ler and GrlA|url= |journal=Journal of Bacteriology|language=en|volume=187|issue=23|pages=7918–7930|doi=10.1128/jb.187.23.7918-7930.2005|issn=0021-9193|pmid=16291665|pmc=1291265}}
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Mechanism
Ler positively regulates the LEE genes by competition with its homolog, H-NS.{{Cite journal|last1=Winardhi|first1=Ricksen S.|last2=Gulvady|first2=Ranjit|last3=Mellies|first3=Jay L.|last4=Yan|first4=Jie|date=2014-05-16|title=Locus of enterocyte effacement-encoded regulator (Ler) of pathogenic Escherichia coli competes off histone-like nucleoid-structuring protein (H-NS) through noncooperative DNA binding|journal=The Journal of Biological Chemistry|volume=289|issue=20|pages=13739–13750|doi=10.1074/jbc.M113.545954|issn=1083-351X|pmc=4022848|pmid=24668810|doi-access=free}} H-NS silences LEE genes via rigid filament structures bound to the DNA that Ler disrupts and replaces through unknown mechanisms.{{Cite journal|last1=Lim|first1=Ci Ji|last2=Lee|first2=Sin Yi|last3=Kenney|first3=Linda J.|last4=Yan|first4=Jie|date=2012|title=Nucleoprotein filament formation is the structural basis for bacterial protein H-NS gene silencing|journal=Scientific Reports|volume=2|pages=509|doi=10.1038/srep00509|issn=2045-2322|pmc=3396134|pmid=22798986|bibcode=2012NatSR...2E.509L}} Though little is known of the mechanism of Ler regulation, Ler interacts with DNA in specific ways. Ler binds DNA non-cooperatively, bends DNA in low concentrations, stiffens it in high concentration, and forms toroidal nucleoprotein complexes along DNA in vivo.{{Cite journal|last1=Mellies|first1=Jay L.|last2=Benison|first2=Gregory|last3=McNitt|first3=William|last4=Mavor|first4=David|last5=Boniface|first5=Chris|last6=Larabee|first6=Frederick J.|date=April 2011|title=Ler of pathogenic Escherichia coli forms toroidal protein-DNA complexes|journal=Microbiology|volume=157|issue=Pt 4|pages=1123–1133|doi=10.1099/mic.0.046094-0|doi-access=free |issn=1465-2080|pmc=3139439|pmid=21212119}}
Regulation
The regulation of Ler and its transcript, ler, is complex and many-fold. The plasmid encoded regulator (per) directly activates the region of the LEE1 operon which encodes Ler. Integration host factor is also a direct activator of ler and binds upstream of its promoter.{{Cite journal|last1=Friedberg|first1=Devorah|last2=Umanski|first2=Tatiana|last3=Fang|first3=Yuan|last4=Rosenshine|first4=Ilan|date=1999-12-01|title=Hierarchy in the expression of the locus of enterocyte effacement genes of enteropathogenic Escherichia coli|journal=Molecular Microbiology|language=en|volume=34|issue=5|pages=941–952|doi=10.1046/j.1365-2958.1999.01655.x|pmid=10594820|issn=1365-2958|doi-access=free}}
Jeannette Barba and her colleagues at the National Autonomous University of Mexico elucidated a positive regulatory loop between Ler, ler, GrlA, and grlRA. GrlA is also a LEE encoded regulator of the LEE pathogenicity island. They found that GrlA activates ler, and that Ler activates grlRA indicating a loop of activation wherein a protein product activates a transcript whose protein product activates the transcript of the original protein. Ler activates grlRA only if H-NS is present, this is not the case for GrlA activation of ler.
Quorum sensing plays a role in Ler regulation. LuxS is an important protein involved in quorum sensing, particularly in the synthesis of autoinducer molecules. Quorum-sensing E. coli regulator A (QseA) is found in LuxS systems and activates transcription of ler. Fis, a nucleoid associated protein essential for EPEC's ability to form attaching and effacing lesions, partly acts through activation of Ler expression.{{Cite journal|last1=Goldberg|first1=M. D.|last2=Johnson|first2=M.|last3=Hinton|first3=J. C. D.|last4=Williams|first4=P. H.|date=2001-08-01|title=Role of the nucleoid-associated protein Fis in the regulation of virulence properties of enteropathogenic Escherichia coli|journal=Molecular Microbiology|language=en|volume=41|issue=3|pages=549–559|doi=10.1046/j.1365-2958.2001.02526.x|pmid=11532124|issn=1365-2958|doi-access=}} BipA, a ribosomal binding GTPase and prolific regulator of EPEC virulence, transcriptionally regulates Ler from an upstream position where it also regulates other genes.{{Cite journal|last1=Grant|first1=Andrew J.|last2=Farris|first2=Michele|last3=Alefounder|first3=Peter|last4=Williams|first4=Peter H.|last5=Woodward|first5=Martin J.|last6=O'Connor|first6=C. David|date=2003-04-01|title=Co-ordination of pathogenicity island expression by the BipA GTPase in enteropathogenic Escherichia coli (EPEC)|journal=Molecular Microbiology|language=en|volume=48|issue=2|pages=507–521|doi=10.1046/j.1365-2958.2003.t01-1-03447.x|pmid=12675808|issn=1365-2958|doi-access=free}}
The Ler protein also represses its own transcript on the LEE1 operon through DNA looping which prevents RNA polymerase from completing transcription.{{Cite journal|last1=Berdichevsky|first1=Tatiana|last2=Friedberg|first2=Devorah|last3=Nadler|first3=Chen|last4=Rokney|first4=Assaf|last5=Oppenheim|first5=Amos|last6=Rosenshine|first6=Ilan|date=2005-01-01|title=Ler Is a Negative Autoregulator of the LEE1 Operon in Enteropathogenic Escherichia coli|url= |journal=Journal of Bacteriology|language=en|volume=187|issue=1|pages=349–357|doi=10.1128/jb.187.1.349-357.2005|issn=0021-9193|pmid=15601719|pmc=538822}}{{Cite journal|last1=Bhat|first1=Abhayprasad|last2=Shin|first2=Minsang|last3=Jeong|first3=Jae-Ho|last4=Kim|first4=Hyun-Ju|last5=Lim|first5=Hyung-Ju|last6=Rhee|first6=Joon Haeng|last7=Paik|first7=Soon-Young|last8=Takeyasu|first8=Kunio|last9=Tobe|first9=Toru|date=2014-06-24|title=DNA looping-dependent autorepression of LEE1 P1 promoters by Ler in enteropathogenic Escherichia coli (EPEC)|journal=Proceedings of the National Academy of Sciences of the United States of America|volume=111|issue=25|pages=E2586–2595|doi=10.1073/pnas.1322033111|issn=1091-6490|pmc=4078829|pmid=24920590|bibcode=2014PNAS..111E2586B|doi-access=free}}