As part of the viral infection cycle, viruses must package their newly replicated genomes for delivery to other host cells. Bacteriophage straight phi29 packages its 6.6-microm long, double-stranded DNA into a 42 x 54 nm capsid by means of a portal complex that hydrolyses ATP. This process is remarkable because entropic, electrostatic and bending energies of the DNA must be overcome to package the DNA to near-crystalline density. Here we use optical tweezers to pull on single DNA molecules as they are packaged, thus demonstrating that the portal complex is a force-generating motor. This motor can work against loads of up to 57 pN on average, making it one of the strongest molecular motors reported to date. Movements of over 5 microm are observed, indicating high processivity. Pauses and slips also occur, particularly at higher forces. We establish the force-velocity relationship of the motor and find that the rate-limiting step of the motor's cycle is force dependent even at low loads. Notably, the packaging rate decreases as the prohead is filled, indicating that an internal force builds up to approximately 50 pN owing to DNA confinement. Our data suggest that this force may be available for initiating the ejection of the DNA from the capsid during infection.

Motors generating mechanical force, powered by the hydrolysis of ATP, translocate doublestranded DNA into preformed capsids (proheads) of bacterial viruses 1,2 and certain animal viruses 3 . Here we describe the motor that packages the double-stranded DNA of the Bacillus subtilis bacteriophage ϕ29 into a precursor capsid. We determined the structure of the head-tail connector-the central component of the ϕ29 DNA packaging motor-to 3.2Å resolution by means of X-ray crystallography. We then fitted the connector into the electron densities of the prohead and of the partially packaged prohead as determined using cryo-electron microscopy and image reconstruction analysis. Our results suggest that the prohead plus dodecameric connector, prohead RNA, viral ATPase and DNA comprise a rotary motor with the head-prohead RNAATPase complex acting as a stator, the DNA acting as a spindle, and the connector as a ball-race. The helical nature of the DNA converts the rotary action of the connector into translation of the DNA.The bacteriophage ϕ29 (Fig. 1) is a 19-kilobase (19-kb) double-stranded DNA (dsDNA) virus with a prolate head and complex structure 4 . The prohead (Fig. 1), into which the DNA is packaged, is about 540Å long and 450Å wide 5 . The ϕ29 connector, a cone-shaped dodecamer of gene product 10 (gp10), occupies the pentagonal vertex at the base of the prohead 5 and is the portal for DNA entry during packaging and DNA ejection during infection 6 . The connector, in association with the oligomeric, ϕ29-encoded prohead RNA (pRNA) and a viral ATPase (gp16), is required for DNA packaging [7][8][9] . However, only the first 120 bases of the 174-base pRNA are essential for packaging 7 the genomic dsDNA with gp3 (DNA-gp3) can be packaged into proheads in about three minutes in vitro (P.J.J., unpublished results). The connector proteins of tailed phages 6 vary in relative molecular mass (M r ) from 36,000 (36K) in ϕ29 to 83K in phage P22, and assemble into oligomers with a central channel. The structure of the isolated ϕ29 connector has been studied by atomic force microscopy 10 and cryo-electron microscopy (cryo-EM) of two-dimensional arrays 11 , immuno-electron microscopy 12 and X-ray crystallography 13,14 .The connector structure, as now determined by X-ray crystallography, can be divided into three, approximately cylindrical regions: the narrow end, the central part, and the wide end, having external radii (Å ) of 33, 47 and 69, respectively (Fig. 2). These regions are respectively 25, 28 and 22Å in height, making the total connector 75Å long. The internal channel has a diameter of about 36Å at the narrow end, increasing to 60Å at the wide end.Comparison with electron microscopy reconstructions 5,11 shows that the narrow end protrudes from the portal vertex of the phage head, is associated with the multimeric pRNA, and binds the lower collar in the mature virus.The electron density of the connector was interpreted in terms of the amino-acid sequence 15 and was confirmed by the two Hg sites (see Methods section)...

Homomeric ring-ATPases perform many vital and varied tasks in the cell, ranging from chromosome segregation to protein degradation. Here we report the first direct observation of the inter-subunit coordination and the step size of such a ring-ATPase, the dsDNA packaging motor in the bacteriophage φ29. Using high-resolution optical tweezers, we find that packaging occurs in increments of 10 bp. Statistical analysis of the preceding dwell times reveals that multiple ATPs bind during each dwell, and application of high force reveals that these 10-bp increments are composed of four 2.5-bp steps. These results indicate that the hydrolysis cycles of the individual subunits are highly coordinated via a mechanism novel for ring-ATPases. In addition, a step size that is a non-integer number of base pairs demands new models for motor-DNA interactions.

A large family of multimeric ATPases are involved in such diverse tasks as cell division, chromosome segregation, DNA recombination, strand separation, conjugation, and viral genome packaging. One such system is the Bacillus subtilis phage phi 29 DNA packaging motor, which generates large forces to compact its genome into a small protein capsid. Here we use optical tweezers to study, at the single-molecule level, the mechanism of force generation in this motor. We determine the kinetic parameters of the packaging motor and their dependence on external load to show that DNA translocation does not occur during ATP binding but is likely triggered by phosphate release. We also show that the motor subunits act in a coordinated, successive fashion with high processivity. Finally, we propose a minimal mechanochemical cycle of this DNA-translocating ATPase that rationalizes all of our findings.

Molecular motors drive genome packaging into preformed procapsids in many dsDNA viruses. Here, we present optical tweezers measurements of single DNA molecule packaging in bacteriophage λ. DNA-gpA-gpNu1 complexes were assembled with recombinant gpA and gpNu1 proteins and tethered to microspheres, and procapsids were attached to separate microspheres. DNA binding and initiation of packaging were observed within a few seconds of bringing these microspheres into proximity in the presence of ATP. The motor was observed to generate greater than 50 picoNewtons (pN) of force, in the same range as observed with bacteriophage ϕ29, suggesting that high force generation is a common property of viral packaging motors. However, at low capsid filling the packaging rate averaged ~600 bp/s, which is 3.5-fold higher than ϕ29, and the motor processivity was also 3-fold higher, with less than one slip per genome length translocated. The packaging rate slowed significantly with increasing capsid filling, indicating a buildup of internal force reaching 14 pN at 86% packaging, in good agreement with the force driving DNA ejection measured in osmotic pressure experiments and calculated theoretically. Taken together, these experiments show that the internal force that builds during packaging is largely available to drive subsequent DNA ejection. In addition, we observed an 80 bp/s dip in the average packaging rate at 30% packaging, suggesting that procapsid expansion occurs at this point following the buildup of an average of 4 pN of internal force. In experiments with a DNA construct longer than the wild-type genome, a sudden acceleration in packaging rate was observed above 90% packaging in many cases, and greater than 100% of the genome length was translocated, suggesting that internal force can rupture the immature procapsid.