12 public questions tagged with this topic.
Polycistronic mRNA contains multiple genes, typically found in prokaryotes, which are translated into different proteins.
Ref: NCERT Class 12 Biology Chapter 5: Molecular Basis of Inheritance Human Genome Project and DNA Fingerprinting
The promoter is the site where RNA polymerase binds to initiate transcription in an operon.
Ref: NCERT Class 12 Biology Chapter 5: Molecular Basis of Inheritance Transcription - Mechanism and RNA Polymerase
The terminator sequence signals the end of transcription in prokaryotes.
Ref: NCERT Class 12 Biology Chapter 5: Molecular Basis of Inheritance Transcription - Mechanism and RNA Polymerase
Prokaryotes have a single RNA polymerase, unlike eukaryotes which have multiple types of RNA polymerases.
Ref: NCERT Class 12 Biology Chapter 5: Molecular Basis of Inheritance Transcription - Mechanism and RNA Polymerase
Prokaryotic mRNA does not undergo extensive post-transcriptional modifications like splicing, unlike eukaryotic mRNA.
Ref: NCERT Class 12 Biology Chapter 5: Molecular Basis of Inheritance Transcription - Mechanism and RNA Polymerase
Prokaryotic mRNA does not undergo extensive post-transcriptional modifications such as splicing, unlike eukaryotic mRNA.
Ref: NCERT Class 12 Biology Chapter 5: Molecular Basis of Inheritance Transcription - Mechanism and RNA Polymerase
Prokaryotic genes lack introns, as prokaryotic mRNA does not undergo splicing.
Ref: NCERT Class 12 Biology Chapter 5: Molecular Basis of Inheritance Transcription - Mechanism and RNA Polymerase
Catabolite repression couples glucose availability to cyclic AMP synthesis via adenylate cyclase. When glucose transport is low, EIIA phosphorylated activates cyclase, raising cAMP levels. Cyclic AMP binds catabolite activator protein CAP, inducing conformational shift allowing dimerization and DNA recognition. The CAP-cAMP complex binds conserved sites near catabolite-sensitive promoters including lac, ara, gal, facilitating RNA polymerase recruitment through alpha-CTD contact. When glucose abundant, cAMP falls, CAP remains inactive, transcription of alternative sugar operons reduced. Thus CAP-cAMP binding specifically signals carbon starvation and glucose limitation, coordinating hierarchical sugar utilization for energy efficiency.
Ref: Lodish et al., Molecular Cell Biology, 9th ed., Chapter 8: Bacterial Transcription Control and CAP-cAMP
Presence of L-arabinose triggers major structural rearrangement within AraC regulatory protein. Sugar molecule binds deep pocket within N-terminal domain forming hydrogen bonds that retract N-terminal arm from dimerization interface, altering interdomain orientation and allosteric communication. Dimer consequently loses high affinity for distal O2 site, abandons loop, and preferentially occupies adjacent half-sites araI1 centered minus 100 and araI2 at minus 50 directly upstream of PBAD promoter. Occupancy of I1-I2 positions AraC activation surface near minus45 facilitating direct interaction with sigma70 subunit and alpha C-terminal domain of RNA polymerase, breaking repressive loop and potently stimulating transcription initiation of araBAD catabolic genes.
Ref: Wikipedia L-arabinose operon – arabinose-bound AraC binds I1 and I2 acting as activator for araBAD expression.
During initial transcription, RNA polymerase undergoes iterative cycles of synthesizing and releasing short RNAs while remaining bound to promoter. Structural blockage occurs because sigma factor region 3.2 loop threads through RNA exit channel, obstructing extension beyond about nine to ten nucleotides. Polymerase synthesizes short oligoribonucleotides two to nine bases that escape into environment as abortive transcripts before productive escape. These ≤10 nucleotide products characteristic of abortive initiation phase. Overcoming this barrier requires conformational change displacing sigma block upon sufficient RNA length and energy, transitioning to elongation competent complex processively synthesizing full-length RNA.
Ref: Watson et al. Chapter 13: Abortive initiation RNA length ≤10 nucleotides; Alberts Chapter 6 Abortive transcripts initiation complex instability
UP element is AT-rich sequence located approximately -40 to -60 upstream of transcription start site, found in strong rRNA and tRNA promoters. Crystallographic studies show C-terminal domain of α subunit, α-CTD connected via flexible linker, contains helix-hairpin motif binding minor groove of UP element AT tracts, providing additional anchoring energy independent of sigma -10 and -35 contacts. Interaction can enhance transcription up to thirty fold. β, σ70, β', ω subunits do not directly contact UP. Presence explains why some promoters lacking perfect consensus still strong due to extra α-CTD mediated recruitment of polymerase holoenzyme.
Ref: Ross et al. Science 1993 UP element α-CTD interaction; Alberts Chapter 6: UP recognized by α-CTD domain structure
Within bacterial holoenzyme, sequence-specific readout of promoter consensus relies predominantly on sigma factor rather than core catalytic subunits. Sigma70 region 4.2 presents helix-turn-helix motif inserting into major groove at -35 element TTGACA, while region 2.4 interacts with -10 Pribnow box TATAAT and captures flipped bases to drive melting. α subunit C-terminal domain may touch UP element AT-rich tract, but -10 and -35 recognition is sigma intrinsic. β and β' subunits shape downstream DNA channel and catalytic Mg2+ but do not determine promoter consensus identity. Exchange of sigma alters promoter preference accordingly.
Ref: Alberts Molecular Biology Cell Chapter 6: Sigma subunit recognizes -10 -35 elements; Watson Chapter 13 Sigma-DNA contacts
