Escherichia coli glycogen genes are organized in a single glgBXCAP transcriptional unit possessing an alternative suboperonic promoter within glgC that directs glgAP expression
Résumé
Although it is generally accepted that E. coli glycogen genes are organized in two tandemly arranged, differentially regulated glgBX and glgCAP operons, RT-PCR analyses carried out in this work showed that E. coli cells possess transcripts comprising the five glgBXCAP genes. glg::lacZY expression analyses in cells lacking the region immediately upstream of the glgB gene revealed an almost total abolishment of glgB, glgX and glgC expression, but only a 50-60% reduction of the wild type glgA and glgP expression levels. Furthermore, similar type of analyses showed that glgA and glgP expression was almost totally abolished in cells lacking glgA upstream sequences including glgC, glgB and the asd-glgB intergenic region upstream of glgB. The overall data thus indicated that E. coli glgBXCAP genes are organized in a single transcriptional unit controlled by promoter sequences occurring upstream of glgB, and that an alternative suboperonic promoter is located within glgC driving expression of glgA and glgP genes. Computer searches for consensus promoters, and analyses of glgB::lacZY and glgA::lacZY expression in cells containing deletions of glgB and glgA upstream sequences identified regions directing glgBXCAP and glgAP expression. 5´ RACE analyses located a glgBXCAP transcription start site 155 bp upstream of the glgB initiation codon, and a glgAP transcription start site 359 bp upstream of the glgA initiation codon. Finally, glg::lacZY expression analyses on cells lacking relA or phoP regulatory genes indicated that both glgBXCAP operon and the suboperonic promoter driving glgAP expression form part of both the RelA and the PhoP-PhoQ regulons.
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