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AKH9 Dynamics of the trp Operon Refs: Sántilan & Mackey, PNAS 98 (4), 1364-69 [2001] I. Rahmim doctoral thesis, Columbia Univ., 1990 Alberts, MBOTC III, pp. 417-419, Lewin, Genes V, 435-7 O v e r v i e w (Santillan) Organization of the trp Operon* • 5 genes code for enzymes: * Cluster of genes controlled by a single (?) feedback regulatory mechanism. The five genes are transcribed as a single mRNA molecule, allowing their expression to be controlled coordinately. There is one promoter. Within the promoter is an operator. Tryptophan repressor can bind to operator and deny access to RNA polymerase. Because repressor binding stops transcription the regulation mode is called negative control. Repressor Terminology Activation (binding) of repressor requires binding of two tryptophan molecules: Multiple RNA polymerases act together on an operon; multiple ribosomes act together on even partially formed mRNAs: Attenuators… act to terminate transcription prematurely. They act before or within the coding region When they are non-functional, transcription continues past attenuation region When they are functional, termination occurs Tryptophan synthesis: chorismate l-tryptophan (the 5 enzymes that make tryptophan) trp E trp D trp C trp B trp A Anthranilate synthase Phosphoribosyl anthranilae transferase Phosphoribosyl isomerase/indoleglycerol phosphate synthase Tryptophan synthetase a Tryptophan synthase b Tryptophan Operon Promoters: p1 (strong): Active repressor binds to trp operator and overlaps p1 with physical exclusion of RNA polymerase p2 (low efficiency): Within distal part of trpD, results in production of mRNAs C, B, and A. Thus, production of trp E and D is differentiated from production of trp C, B, and A. Additionally, deactivation and degradation of mRNA CBA is much slower than for mRNA ED. Just before trpE transcription is regulated through attenuation. At low tryptophan levels, reduces by 8 Enzymatic Control of Tryptophan Levels: Production: Anthranilate synthase inhibited up to 100% by tryptophan. Interconversion: Tryptophanase (tna) catalyzes tryptophanindole interconversion. Transport Control of Tryptophan levels: aro P product facilitates transport of aromatic amino acids. mtr product facilitates tryptophan transport. (mtr is activated with phenylalanine present, tryptophan absent.) Product of tyrR modulates expression of aroP and mtr. Transport dynamics have not been studied – we don’t know the transporter kinetic coefficients. Goodwin’s compartmental triad in bacterial gene expression: d [mRNA] kmRNA r [ gene] kd ,mRNA [mRNA], dt d [enzyme] kenzyme [mRNA] kd ,enzyme [enzyme], dt d [ product ] k product [enzyme] kd , product [ product ] dt r (efficiency of feedback repression by product) = 1 product 1 K D , product n (a negative-going Hill coefficient) Hill Functions: F, F+, F1.00 0.75 Y 0.50 0.25 0.00 0.00 x , =1 x x 4 x 4 1.00 2.00 X 4 , =1 4 x 4 4 , K =1 All of these can be written with the non-dimensional variable, x / Overall Model for trp Operon About concentrations: • Molecules per cell (with a 'standard' cell size). Intuitive but not usually measured. • Standard chemical concentrations, e.g. moles/ml, etc. Normal measurement. • Stochasticity issues. 50 molecules per cell of volume 1m3 ~ 0.1 M. The Effect of Cell Expansion on Concentration • Expanding (dividing) cultures lose concentration even without degradation – • amount in one cell is unaffected until division, and is then suddenly halved. • concentration decreases steadily up to, during, and after division. • Intracellular heterogeneous kinetics are affected by concentration, not amount. Allowing for Volume Expansion Without Expansion: dc generation Vkd c dt rate With expansion: V dc dV generation +c Vkd c dt dt rate which can be written: V dc generation dV generation d lnV V Vkd c c V kd c dt rate dt rate dt d lnV The term is frequently designated as . Expansion dt increases the apparent degradation rate. It can be the most significant factor affecting metabolite concentrion during growth! Cell Volume Vc (in liters) = 4 10 -15 60 e -1 where is the specific growth rate, min . mRNA Equations d [mRNA ED] v p1 r a gT dt (kd ,mRNA ED )[mRNA ED] d [mRNA CBA] v p1 r a gT v p 2 gT dt (kd ,mRNA CBA )[mRNA CBA] Repression: [apoR] [tryp] apoR tryp, K r1 apoR+ tryp [apoR tryp] [apoR tryp] [tryp] holo R, K r1 apoR tryp tryp [holo R] [holo R] [operator] bound repressor, K r2 holoR free operator [bound repressor] "free" + "occupied" operators = g T ; apoR + apoR tryp + holoR + bound repressors = r0 ; "occupied" trp operators = repressors bound to trp operators. Intracellular Concentration* of Bound trp Repressors: B B 4 gT r0 2 2 where B r0 gT K r 2 (1 A A2 ) , and K r1 A [tryp] __________________________________ * = molecules per cell/ (cell volume NA) Finally, for repression: conc'n of free trp operators r gT conc'n of occupied trp operators 1 gT conc'n of bound trp repressors 1 gT Attenuation trp leader region: Transcription is stalled on the pause site until a ribosome arrives at base 27 How long does RNA polymerase take to move from the pause site to the potential termination site, i.e. what is T? How far does the ribosome travel during time T? Simulation: RNA polymerase transcribes at an average rate of 42 nt/sec, but follows Poisson distribution. Ribosome adds residues at a corresponding average rate of 14/sec, but dependent on amino acid availability. Rate is maximum at high, and zero at zero, concentration. vribosome [ A] 14 (residues/sec.) K a [ A] Assume tryptophan concentration controlling. Monte Carlo Simulation Results: a 3.0 [tryp] / K a 0.12 0.4 [tryp] / K a Other Coefficients Needed • mRNA Degradation Rates (t1/2 values) • Transcription Initiation Rates Enzyme Balances dz3 vz 3[mRNA ED] (kd , protein ) z3 dt dz4 vz 4 [mRNA CBA] (kd , protein ) z4 dt (kd , protein ) z3 (kd , protein ) z4 vz 3 ; vz 4 [mRNA ED] [mRNA CBA] Biosynthesis Pathway K I , ASase d [anth] Vmax, ASase [ ASase] dt K I , ASase [tryp] [anth] Vmax, PRTase [ PRTase] [anth] K PRTase [anth] and similarly for CdRP and tryp (with the addition of the utilization term, - U c , in the tryp equation. The term in red represents inhibition of the first enzyme by tryp. Redux: Overall Model for trp Operon