BIOL2010 - Transcription Lecture 2 by D. Doyle PDF

BIOL2010 – Transcription Lecture 2 – Transcription in prokaryotes Dr Declan A. Doyle E-mail: [email protected]  Similar Transcriptional Control Control of transcription is the major way protein expression is controlled More...

BIOL2010 – Transcription Lecture 2 – Transcription in prokaryotes Dr Declan A. Doyle E-mail: [email protected]  Similar Transcriptional Control Control of transcription is the major way protein expression is controlled More protein required mRNA synthesis Protein synthesis Less protein required mRNA synthesis Protein synthesis  Transcriptional Control in Bacteria A) Constitutive genes – housekeeping genes B) Regulated genes e.g. changes in food sources Switches on genes that encode enzymes which are needed to metabolise that sugar e.g. changes in environmental stresses (pH and temp) Switches on genes that encode proteins which help the bacterium survive  Transcriptional Control in Bacteria Operons Genes encoding for proteins in the same pathway are located adjacent to one other and controlled as a single unit that is transcribed into a polycistronic RNA - No introns DNA Promoter RNA Pol Gene A Gene B Gene C mRNA AUG TAA AUG TAA AUG TAA Protein A Protein B Protein C  Transcriptional Control in Bacteria Lac operons e.g. changes in lactose concentration Switches on genes that encode enzymes which are needed to metabolise that sugar lac operon E-coli DNA Promoter chromosom lacZ lacY lacA e lac terminator -Galactosidase Permease Transacetylase LacA’s cellular role may be to detoxify non- metabolisable pyranosides by acetylating them and preventing their re-entry into the cell.  Bacterial Promoters  Control of transcription most commonly occurs at transcription initiation  Promoters are the sites of transcription initiation in the DNA Start point +1 Coding DNA 5’ strand A 3’ 3’ T 5’ Template DNA strand Transcription Promoter A 3’ RNA -ve upstream +ve downstream  Bacterial Promoters  Promoters are recognised by RNA polymerase by having a consensus (common pattern of) DNA sequence - hexamer (6bp) at -35 and a TATAAT sequence at -10 - asymmetric (only in 1 DNA strand), hence RNA polymerase knows which way to go! Pribnow box Start point -35 sequence -10 sequence +1 Coding DNA strand TTGACA TATAAT A 3’ 5’ 3’ AACTGT ATATTA T 5’ Template DNA strand Transcription Promoter A 3’ RNA -ve upstream +ve downstream Bacterial Promoters Consensus sequences  Is defined by aligning all known examples as to maximise homology  Down-mutations to decrease promoter efficiency usually decrease conformance to the consensus sequence  Up-mutations have the opposite effect Protein over-expression in Escherichia coli triggers adaptation analogous to antimicrobial resistance Down-mutation example LW – large white phenotype LG – large green SCV – small colony variant  Protein over-expression in Escherichia coli triggers adaptation analogous to antimicrobial resistance Down-mutation example LW, LG and SCV phenotypes are the