A single-molecule approach to visualize the unwinding activity of DNA helicases

Almost all aspects of DNA metabolism involve separation of double-stranded DNA catalyzed by helicases. Observation and measurement of the dynamics of these events at the single-molecule level provide important mechanistic details of helicase activity and give the opportunity to probe aspects that are not revealed in bulk solution measurements. The assay, presented here, provides information about helicase unwinding rates and processivity. Visualization is achieved by using a fluorescent single-stranded DNA-binding protein (SSB), which allows the time course of individual DNA unwinding events to be observed using total internal reflection fluorescence microscopy. Observation of a prototypical helicase, Bacillus subtilis AddAB, shows that the unwinding process consists of bursts of unwinding activity, interspersed with periods of pausing.

The AddAB helicase-nuclease catalyses rapid and processive DNA unwinding using a single Superfamily 1A motor domain

The oligomeric state of Superfamily I DNA helicases is the subject of considerable and ongoing debate. While models based on crystal structures imply that a single helicase core domain is sufficient for DNA unwinding activity, biochemical data from several related enzymes suggest that a higher order oligomeric species is required. In this work we characterize the helicase activity of the AddAB helicase–nuclease, which is involved in the repair of double-stranded DNA breaks in Bacillus subtilis. We show that the enzyme is functional as a heterodimer of the AddA and AddB subunits, that it is a rapid and processive DNA helicase, and that it catalyses DNA unwinding using one single-stranded DNA motor of 3′→5′ polarity located in the AddA subunit. The AddB subunit contains a second putative ATP-binding pocket, but this does not contribute to the observed helicase activity and may instead be involved in the recognition of recombination hotspot sequences.

Fluorescent biosensors to investigate helicase activity

ATP-driven translocation of helicases along DNA can be assayed in several ways. Reagentless biosensors, based on fluorophore-protein adducts, provide convenient ways for real-time assays of both the separation of dsDNA and the hydrolysis of ATP. Single-stranded DNA can be assayed using a modified single-stranded DNA-binding protein (SSB), and phosphate production during ATP hydrolysis can be measured by a modified phosphate-binding protein. Advantages and limitations of these approaches are compared with those of other types of measurements.

A fluorescent, reagentless biosensor for ADP based on tetramethylrhodamine-labeled ParM

Fluorescence assays for ADP detection are of considerable current interest, both in basic research and in drug discovery, as they provide a generic method for measuring the activity of ATPases and kinases. The development of a novel fluorescent biosensor is described that is based on a tetramethylrhodamine-labeled, bacterial actin homologue, ParM. The design of the biosensor takes advantage of the large conformational change of ParM on ADP binding and the strong quenching of the tetramethylrhodamine fluorescence by stacking of the dye. ParM was labeled with two tetramethylrhodamines in close proximity, whereby the fluorophores are able to interact with each other. ADP binding alters the distance and relative orientation of the tetramethylrhodamines, which leads to a change in this stacking interaction and so in the fluorescence intensity. The final ADP biosensor shows ∼15-fold fluorescence increase in response to ADP binding. It has relatively weak affinity for ADP (K_d = 30 μM), enabling it to be used at substoichiometric concentrations relative to ADP, while reporting ADP concentration changes in a wide range around the K_d value, namely, submicromolar to tens of micromolar. The biosensor strongly discriminates against ATP (>100-fold), allowing ADP detection against a background of millimolar ATP. At 20 °C, the labeled ParM binds ADP with a rate constant of 9.5 × 10⁴ M⁻¹ s⁻¹ and the complex dissociates at 2.9 s⁻¹. Thus, the biosensor is suitable for real-time measurements, and its performance in such assays is demonstrated using a sugar kinase and a mammalian protein kinase.

Visualizing helicases unwinding DNA at the single molecule level

DNA helicases are motor proteins that catalyze the unwinding of double-stranded DNA into single-stranded DNA using the free energy from ATP hydrolysis. Single molecule approaches enable us to address detailed mechanistic questions about how such enzymes move processively along DNA. Here, an optical method has been developed to follow the unwinding of multiple DNA molecules simultaneously in real time. This was achieved by measuring the accumulation of fluorescent single-stranded DNA-binding protein on the single-stranded DNA product of the helicase, using total internal reflection fluorescence microscopy. By immobilizing either the DNA or helicase, localized increase in fluorescence provides information about the rate of unwinding and the processivity of individual enzymes. In addition, it reveals details of the unwinding process, such as pauses and bursts of activity. The generic and versatile nature of the assay makes it applicable to a variety of DNA helicases and DNA templates. The method is an important addition to the single-molecule toolbox available for studying DNA processing enzymes.

Mechanism of interaction between single-stranded DNA binding protein and DNA

A single-stranded DNA binding protein (SSB), labeled with a fluorophore, interacts with single-stranded DNA (ssDNA), giving a 6-fold increase in fluorescence. The labeled protein is the adduct of the G26C mutant of the homotetrameric SSB from Escherichia coli and a diethylaminocoumarin {N-[2-(iodoacetamido)ethyl]-7-diethylaminocoumarin-3-carboxamide}. This adduct can be used to assay production of ssDNA during separation of double-stranded DNA by helicases. To use this probe effectively, as well as to investigate the interaction between ssDNA and SSB, the fluorescent SSB has been used to develop the kinetic mechanism by which the protein and ssDNA associate and dissociate. Under conditions where ∼70 base lengths of ssDNA wrap around the tetramer, initial association is relatively simple and rapid, possibly diffusion-controlled. The kinetics are similar for a 70-base length of ssDNA, which binds one tetramer, and poly(dT), which could bind several. Under some conditions (high SSB and/or low ionic strength), a second tetramer binds to each 70-base length, but at a rate 2 orders of magnitude slower than the rate of binding of the first tetramer. Dissociation kinetics are complex and greatly accelerated by the presence of free wild-type SSB. The main route of dissociation of the fluorescent SSB·ssDNA complex is via association first with an additional SSB and then dissociation. Comparison of binding data with different lengths of ssDNA gave no evidence of cooperativity between tetramers. Analytical ultracentrifugation was used to determine the dissociation constant for labeled SSB₂·dT₇₀ to be 1.1 μM at a high ionic strength (200 mM NaCl). Shorter lengths of ssDNA were tested for binding: only when the length is reduced to 20 bases is the affinity significantly reduced.

A biosensor for fluorescent determination of ADP with high time resolution

Nearly every cellular process requires the presence of ATP. This is reflected in the vast number of enzymes like kinases or ATP hydrolases, both of which cleave the terminal phosphate from ATP, thereby releasing ADP. Despite the fact that ATP hydrolysis is one of the most fundamental reactions in biological systems, there are only a few methods available for direct measurements of enzymatic-driven ATP conversion. Here we describe the development of a reagentless biosensor for ADP, the common product of all ATPases and kinases, which allows the real-time detection of ADP, produced enzymatically. The biosensor is derived from a bacterial actin homologue, ParM, as protein framework. A single fluorophore (a diethylaminocoumarin), attached to ParM at the edge of the nucleotide binding site, couples ADP binding to a >3.5-fold increase in fluorescence intensity. The labeled ParM variant has high affinity for ADP (0.46 μm) and a fast signal response, controlled by the rate of ADP binding to the sensor (0.65 μm⁻¹s⁻¹). Amino acids in the active site were mutated to reduce ATP affinity and achieve a >400-fold discrimination against triphosphate binding. A further mutation ensured that the final sensor did not form filaments and, as a consequence, has extremely low ATPase activity. The broad applicability of N-[2-(1-maleimidyl)ethyl]-7-diethylaminocoumarin-3-carboxamide (MDCC)-ParM as a sensitive probe for ADP is demonstrated in real-time kinetic assays on two different ATPases and a protein kinase.

The ATPase cycle of PcrA helicase and its coupling to translocation on DNA

The superfamily 1 bacterial helicase PcrA has a role in the replication of certain plasmids, acting with the initiator protein (RepD) that binds to and nicks the double-stranded origin of replication. PcrA also translocates single-stranded DNA with discrete steps of one base per ATP hydrolyzed. Individual rate constants have been determined for the DNA helicase PcrA ATPase cycle when bound to either single-stranded DNA or a double-stranded DNA junction that also has RepD bound. The fluorescent ATP analogue 2'(3')-O-(N-methylanthraniloyl)ATP was used throughout all experiments to provide a complete ATPase cycle for a single nucleotide species. Fluorescence intensity and anisotropy stopped-flow measurements were used to determine rate constants for binding and release. Quenched-flow measurements provided the kinetics of the hydrolytic cleavage step. The fluorescent phosphate sensor MDCC-PBP was used to measure phosphate release kinetics. The chemical cleavage step is the rate-limiting step in the cycle and is essentially irreversible and would result in the bound ATP complex being a major component at steady state. This cleavage step is greatly accelerated by bound DNA, producing the high activation of this protein compared to the protein alone. The data suggest the possibility that ADP is released in two steps, which would result in bound ADP also being a major intermediate, with bound ADP.P(i) being a very small component. It therefore seems likely that the major transition in structure occurs during the cleavage step, rather than P(i) release. ATP rebinding could then cause reversal of this structural transition. The kinetic mechanism of the PcrA ATPase cycle is very little changed by potential binding to RepD, supporting the idea that RepD increases the processivity of PcrA by increasing affinity to DNA rather than affecting the enzymatic properties per se.

PcrA helicase tightly couples ATP hydrolysis to unwinding double-stranded DNA, modulated by the initiator protein for plasmid replication, RepD

The plasmid replication initiator protein, RepD, greatly stimulates the ability of the DNA helicase, PcrA, to unwind plasmid lengths of DNA. Unwinding begins at oriD, the double-stranded origin of replication that RepD recognizes and covalently binds to initiate replication. Using a combination of plasmids containing oriD and oligonucleotide structures that mimic parts of oriD, the kinetics of DNA nicking and separation have been determined, along with the coupling ratio between base separation and ATP hydrolysis. At 30 °C, the rate of nicking is 1.0 s⁻¹, and translocation is ∼30 bp s⁻¹. During translocation, the coupling ratio is one ATP hydrolyzed per base pair separated, the same as the value previously reported for ATP hydrolyzed per base moved by PcrA along single-stranded DNA. The data suggest that processivity is high, such that several thousand base-pair plasmids are unwound by a single molecule of PcrA. In the absence of RepD, a single PcrA is unable to separate even short lengths (10 to 40 bp) of double stranded DNA.

Mechanistic basis of 5'-3' translocation in SF1B helicases

Superfamily 1B (SF1B) helicases translocate in a 5'-3' direction and are required for a range of cellular activities across all domains of life. However, structural analyses to date have focused on how SF1A helicases achieve 3'-5' movement along nucleic acids. We present crystal structures of the complex between the SF1B helicase RecD2 from Deinococcus radiodurans and ssDNA in the presence and absence of an ATP analog. These snapshots of the reaction pathway reveal a nucleotide binding-induced conformational change of the two motor domains that is broadly reminiscent of changes observed in other SF1 and SF2 helicases. Together with biochemical data, the structures point to a step size for translocation of one base per ATP hydrolyzed. Moreover, the structures also reveal a mechanism for nucleic acid translocation in the 5'-3' direction by SF1B helicases that is surprisingly different from that of 3'-5' translocation by SF1A enzymes, and explains the molecular basis of directionality.

Time course and strain dependence of ADP release during contraction of permeabilized skeletal muscle fibers

A phosphorylated, single cysteine mutant of nucleoside diphosphate kinase, labeled with N-[2-(iodoacetamido)ethyl]-7-diethylaminocoumarin-3-carboxamide (P approximately NDPK-IDCC), was used as a fluorescence probe for time-resolved measurement of changes in [MgADP] during contraction of single permeabilized rabbit psoas fibers. The dephosphorylation of the phosphorylated protein by MgADP occurs within the lattice environment of permeabilized fibers with a second-order rate constant at 12 degrees C of 10(5) M(-1) s(-1). This dephosphorylation is accompanied by a change in coumarin fluorescence. We report the time course of P approximately NDPK-IDCC dephosphorylation during the period of active isometric force redevelopment after quick release of fiber strain at pCa(2+) of 4.5. After a rapid length decrease of 0.5% was applied to the fiber, the extra NDPK-IDCC produced during force recovery, above the value during the approximately steady state of isometric contraction, was 2.7 +/- 0.6 microM and 4.7 +/- 1.5 microM at 12 and 20 degrees C, respectively. The rates of P approximately NDPK-IDCC dephosphorylation during force recovery were 28 and 50 s(-1) at 12 and 20 degrees C, respectively. The time courses of isometric force and P approximately NDPK-IDCC dephosphorylation were simulated using a seven-state reaction scheme. Relative isometric force was modeled by changes in the occupancy of strongly bound A.M.ADP.P(i) and A.M.ADP states. A strain-sensitive A.M.ADP isomerization step was rate-limiting (3-6 s(-1)) in the cross-bridge turnover during isometric contraction. At 12 degrees C, the A.M.ADP.P(i) and the pre- and postisomerization A.M.ADP states comprised 56%, 38%, and 7% of the isometric force-bearing AM states, respectively. At 20 degrees C, the force-bearing A.M.ADP.P(i) state was a lower proportion of the total force-bearing states (37%), whereas the proportion of postisomerization A.M.ADP states was higher (19%). The simulations suggested that release of cross-bridge strain caused rapid depopulation of the preisomerization A.M.ADP state and transient accumulation of MgADP in the postisomerization A.M.ADP state. Hence, the strain-sensitive isomerization of A.M.ADP seems to explain the rate of change of P approximately NDPK-IDCC dephosphorylation during force recovery. The temperature-dependent isometric distribution of myosin states is consistent with the previous observation of a small decrease in amplitude of the P(i) transient during force recovery at 20 degrees C and the current observation of an increase in amplitude of the ADP-sensitive NDPK-IDCC transient.

Switch 1 mutation S217A converts myosin V into a low duty ratio motor

We have determined the kinetic mechanism and motile properties of the switch 1 mutant S217A of myosin Va. Phosphate dissociation from myosin V-ADP-Pi (inorganic phosphate) and actomyosin V-ADP-Pi and the rate of the hydrolysis step (myosin V-ATP-->myosin V-ADP-Pi) were all approximately 10-fold slower in the S217A mutant than in wild type (WT) myosin V, resulting in a slower steady-state rate of basal and filamentous actin (actin)-activated ATP hydrolysis. Substrate binding and ADP dissociation kinetics were all similar to or slightly faster in S217A than in WT myosin V and mechanochemical gating of the rates of dissociation of ADP between trail and lead heads is maintained. The reduction in the rate constants of the hydrolysis and phosphate dissociation steps reduces the duty ratio from approximately 0.85 in WT myosin V to approximately 0.25 in S217A and produces a motor in which the average run length on actin at physiological concentrations of ATP is reduced 10-fold. Thus we demonstrate that, by mutational perturbation of the switch 1 structure, myosin V can be converted into a low duty ratio motor that is processive only at low substrate concentrations.

Direct observation of the mechanochemical coupling in myosin Va during processive movement

Myosin Va transports intracellular cargoes along actin filaments in cells. This processive, two-headed motor takes multiple 36-nm steps in which the two heads swing forward alternately towards the barbed end of actin driven by ATP hydrolysis. The ability of myosin Va to move processively is a function of its long lever arm, the high duty ratio of its kinetic cycle and the gating of the kinetics between the two heads such that ADP release from the lead head is greatly retarded. Mechanical studies at the multiple- and the single-molecule level suggest that there is tight coupling (that is, one ATP is hydrolysed per power stroke), but this has not been directly demonstrated. We therefore investigated the coordination between the ATPase mechanism of the two heads of myosin Va and directly visualized the binding and dissociation of single fluorescently labelled nucleotide molecules, while simultaneously observing the stepping motion of the fluorescently labelled myosin Va as it moved along an actin filament. Here we show that preferential ADP dissociation from the trail head of mouse myosin Va is followed by ATP binding and a synchronous 36-nm step. Even at low ATP concentrations, the myosin Va molecule retained at least one nucleotide (ADP in the lead head position) when moving. Thus, we directly demonstrate tight coupling between myosin Va movement and the binding and dissociation of nucleotide by simultaneously imaging with near nanometre precision.

Kinetics of ADP dissociation from the trail and lead heads of actomyosin V following the power stroke

Myosin V is a cellular motor protein, which transports cargos along actin filaments. It moves processively by 36-nm steps that require at least one of the two heads to be tightly bound to actin throughout the catalytic cycle. To elucidate the kinetic mechanism of processivity, we measured the rate of product release from the double-headed myosin V-HMM using a new ATP analogue, 3'-(7-diethylaminocoumarin-3-carbonylamino)-3'-deoxy-ATP (deac-aminoATP), which undergoes a 20-fold increase in fluorescence emission intensity when bound to the active site of myosin V (Forgacs, E., Cartwright, S., Kovács, M., Sakamoto, T., Sellers, J. R., Corrie, J. E. T., Webb, M. R., and White, H. D. (2006) Biochemistry 45, 13035-13045). The kinetics of ADP and deac-aminoADP dissociation from actomyosin V-HMM, following the power stroke, were determined using double-mixing stopped-flow fluorescence. These used either deac-aminoATP as the substrate with ADP or ATP chase or alternatively ATP as the substrate with either a deac-aminoADP or deac-aminoATP chase. Both sets of experiments show that the observed rate of ADP or deac-aminoADP dissociation from the trail head of actomyosin V-HMM is the same as from actomyosin V-S1. The dissociation of ADP from the lead head is decreased by up to 250-fold.

A biosensor for inorganic phosphate using a rhodamine-labeled phosphate binding protein

A novel biosensor for inorganic phosphate (Pi) has been developed based on the phosphate binding protein of Escherichia coli. Two cysteine mutations were introduced and labeled with 6-iodoacetamidotetramethylrhodamine. When physically close to each other and correctly oriented, two rhodamine dyes interact to form a noncovalent dimer. In this state, they have little or no fluorescence, unlike the high fluorescence intensity of monomeric rhodamine. The labeling sites were so placed that the distance and orientation between the rhodamines change as a consequence of the conformational change associated with Pi binding. This movement alters the extent of interaction between the dyes. The best mutant of those tested (A17C, A197C) gives rise on average to approximately 18-fold increase in fluorescence intensity as Pi binds. The kinetics of interaction with Pi were measured at 10 degrees C. Under these conditions, the fluorescence increase associated with Pi binding has a maximum rate of 267 s-1. The Pi dissociation rate is 6.6 s-1, and the dissociation constant is 70 nM. An application of the sensor is demonstrated for measuring ATP hydrolysis in real time as a helicase moves along DNA. Advantages of the new sensor are discussed, both in terms of the use of a rhodamine fluorophore and the potential of this double labeling strategy.

Development of fluorescent biosensors for probing the function of motor proteins

Biosensors are becoming widely used both in basic research and in screening assays and reagentless sensors with fluorescent reporter groups attached to proteins form one class. This article describes the development of sensors for two small molecules, driven in particular by the need for high sensitivity and time resolution to probe mechanistic aspects of ATP-coupled motor proteins. The biosensors are for the products of the ATPase reaction, ADP and inorganic phosphate. The interplay between the possibilities for design and understanding the mechanism of the signal are discussed. Examples are described of how these sensors have been applied to understanding myosin and helicase motors.

Kinetic mechanism of myosinV-S1 using a new fluorescent ATP analogue

We have used a new fluorescent ATP analogue, 3'-(7-diethylaminocoumarin-3-carbonylamino)-3'-deoxyadenosine-5'-triphosphate (deac-aminoATP), to study the ATP hydrolysis mechanism of the single headed myosinV-S1. Our study demonstrates that deac-aminoATP is an excellent substrate for these studies. Although the deac-amino nucleotides have a low quantum yield in free solution, there is a very large increase in fluorescence emission ( approximately 20-fold) upon binding to the myosinV active site. The fluorescence emission intensity is independent of the hydrolysis state of the nucleotide bound to myosinV-S1. The very good signal-to-noise ratio that is obtained with deac-amino nucleotides makes them excellent substrates for studying expressed proteins that can only be isolated in small quantities. The combination of the fast rate of binding and the favorable signal-to-noise ratio also allows deac-nucleotides to be used in chase experiments to determine the kinetics of ADP and Pi dissociation from actomyosin-ADP-Pi. Although phosphate dissociation from actomyosinV-ADP-Pi does not itself produce a fluorescence signal, it produces a lag in the signal for deac-aminoADP dissociation. The lag provides direct evidence that the principal pathway of product dissociation from actomyosinV-ADP-Pi is an ordered mechanism in which phosphate precedes ADP. Although the mechanism of hydrolysis of deac-aminoATP by (acto)myosinV-S1 is qualitatively similar to the ATP hydrolysis mechanism, there are significant differences in some of the rate constants. Deac-aminoATP binds 3-fold faster to myosinV-S1, and the rate of deac-aminoADP dissociation from actomyosinV-S1 is 20-fold slower. Deac-aminoATP supports motility by myosinV-HMM on actin at a rate consistent with the slower rate of deac-aminoADP dissociation.

The mechanism of smooth muscle caldesmon-tropomyosin inhibition of the elementary steps of the actomyosin ATPase

Caldesmon is a component of smooth muscle thin filaments that inhibits the actomyosin ATPase via its interaction with actin-tropomyosin. We have performed a comprehensive transient kinetic characterization of the actomyosin ATPase in the presence of smooth muscle caldesmon and tropomyosin. At physiological ratios of caldesmon to actin (1 caldesmon/7 actin monomers) actomyosin ATPase is inhibited by about 75%. Inhibitory caldesmon concentrations had little effect upon the rate of S1 binding to actin, actin-S1 dissociation by ATP, and dissociation of ADP from actin-S1 x ADP; however the rate of phosphate release from the actin-S1 x ADP x P(i) complex was decreased by more than 80%. In addition the transient of phosphate release displayed a lag of up to 200 ms. The presence of a lag phase indicates that a step on the pathway prior to phosphate release has become rate-limiting. Premixing the actin-tropomyosin filaments with myosin heads resulted in the disappearance of the lag phase. We conclude that caldesmon inhibition of the rate of phosphate release is caused by the thin filament being switched by caldesmon to an inactive state. The active and inactive states correspond to the open and closed states observed in skeletal muscle thin filaments with no evidence for the existence of a third, blocked state. Taken together these data suggest that at physiological concentrations, caldesmon controls the isomerization of the weak binding complex to the strong binding complex, and this causes the inhibition of the rate of phosphate release. This inhibition is sufficient to account for the inhibition of the steady state actomyosin ATPase by caldesmon and tropomyosin.

Mechanism of translocation and kinetics of DNA unwinding by the helicase RecG

RecG is a DNA helicase involved in the repair of damage at a replication fork and catalyzes the reversal of the fork to a point beyond the damage in the template strand. It unwinds duplex DNA in reactions that are coupled to ATP hydrolysis. The kinetic mechanism of duplex DNA unwinding by RecG was analyzed using a quantitative fluorescence assay based on the process of contact quenching between Cy3 and Dabcyl groups attached to synthetic three-way DNA junctions. The data show that the protein moves at a rate of 26 bp s(-1) along the duplex DNA during the unwinding process. RecG ATPase activity during translocation indicates a constant rate of 7.6 s(-1), measured using a fluorescent phosphate sensor, MDCC-PBP. These two rates imply a movement of approximately 3 bp per ATP hydrolyzed. We demonstrate in several trapping experiments that RecG remains attached to DNA after translocation to the end of the arm of the synthetic DNA junction. ATPase activity continues after translocation is complete. Dissociation of RecG from the product DNA occurs only very slowly, suggesting strong interactions between them. The data support the idea that interactions of the duplex template arm with the protein are the major sites of binding and production of translocation.

Bipolar DNA Translocation Contributes to Highly Processive DNA Unwinding by RecBCD Enzyme

We recently demonstrated that the RecBCD enzyme is a bipolar DNA helicase that employs two single-stranded DNA motors of opposite polarity to drive translocation and unwinding of duplex DNA. We hypothesized that this organization may explain the exceptionally high rate and processivity of DNA unwinding catalyzed by the RecBCD enzyme. Using a stopped-flow dye displacement assay for unwinding activity, we test this idea by analyzing mutant RecBCD enzymes in which either of the two helicase motors is inactivated by mutagenesis. Like the wild-type RecBCD enzyme, the two mutant proteins maintain the ability to bind tightly to blunt duplex DNA ends in the absence of ATP. However, the rate of forward translocation for the RecB motor-defective enzyme is only approximately 30% of the wild-type rate, whereas for the RecD motor-defective enzyme, it is approximately 50%. More significantly, the processivity of translocation is substantially reduced by approximately 25- and 6-fold for each mutant enzyme, respectively. Despite retaining the capacity to bind blunt dsDNA, the RecB-mutant enzyme has lost the ability to unwind DNA unless the substrate contains a short 5'-terminated single-stranded DNA overhang. The consequences of this observation for the architecture of the single-stranded DNA motors in the initiation complex are discussed.