Scanning Transmission Electron Microscopy Facility
Joe Wall:  Historical Background of the BNL STEM
Overview The STEM (Scanning Transmission Electron Microscope) concept was pioneered by von Ardenne at the same time as the early development of the TEM. However his efforts were limited by the low brightness of available sources, lack of suitable electronics and poor recording media. Development of the field emission electron gun by Crewe, et al. solved the first problem. Commercially available electronics components provided most of the electronics needs. The annular detector dark field geometry developed by Crewe and Wall made possible imaging of unstained DNA[1] and single heavy atoms[2]. Initial biological results by Wall[3] indicated that specimen preparation and radiation damage were the main limiting factors in STEM imaging of biological specimens, but that with suitable precautions quantitative measurements could answer important questions. Even at that early stage it was possible to show that filamentous viruses were single cylinders with M/L of ~1.6 kDa/Å and "split" structures were actually side by side aggregates of two particles rather than separate strands of a single virus.

The early progress with the STEM at Chicago attracted the attention of John Blewett and Elliott Shaw at Brookhaven who obtained funding from NSF, NIH and DOE in 1971 to develop a STEM Facility along the lines of already-functioning user facilities at BNL. The initial plan was to copy one of the Chicago designs and M. Isaacson was recruited to oversee the construction. When Isaacson returned to Chicago in 1973, J. Wall was recruited with the understanding that design modifications would be considered (not permitted in the original plan). By then it was clear that specimen preparation was severely limiting the quality of results. Therefore a new design of the lens, specimen changer and detectors was developed, emphasizing optimum conditions for the specimen. The present external freeze dryer and vacuum transfer cartridges were part of that complete package. Optics of the STEM were optimized to provide detector acceptance angles and magnification independent of specimen position in the lens gap. All of these features have proven their importance since the official opening of the Facility in October 1977. The STEM remains in the forefront of biological electron microscopes in reliability, productivity and performance.

Work using the STEM at Brookhaven began with a careful study of the first premise of the Facility, namely that single heavy atoms could serve as specific labels for biological specimens. It was determined that these predictions were overly optimistic and that a dose of at least 1000 el/Å2 was needed for adequate S/N (Signal to Noise Ratio)[4], whereas dose-response measurements for mass loss and loss of resolution on freeze dried specimens set a useful imaging limit at 10-100 el/Å2 with the specimen cooled to -140C. Furthermore, the heavy atoms were found to move significantly from one scan to the next [5],[6]. Early experiments with S. Lippard and J. Barton (then at Columbia) confirmed the low label S/N for filamentous virus fd which had each of the 5,000 coat protein subunits labeled with platinum. A promising alternative was TAMM (tetra-mercuri methane) which had 4 mercury atoms within a 3Å radius. This behaved well biochemically, giving 100% labeling, but decomposed in the electron beam[7]. A student of Lippard's, J. Lipka, came to Brookhaven to work on a variety of promising tungsten clusters.

References 1. Crewe, A. V., & Wall, J. S. (1970).
A scanning microscopy with 5 Å resolution.
J. Mol. Biol. 48, 375.

2. Crewe, A. V., Wall, J. S., & Langmore, J. (1970).
Visibility of single atoms.
Science 168, 1338.

3. Wall, J. S. (1971).
A high resolution scanning microscope for the study of single biological molecules.
PhD, University of Chicago.

4. Wall, J. S. (1978-1979).
Limits on visibility of single heavy atoms in the scanning transmission electron microscopy - an experimental study.
Chemica Scripta 14, 271-278.

5. Hainfeld, J., & Wall, J. (1978).
A walking platinum complex.
In Electron Microscopy, 1 (pp. 248-249).

6. Wall, J., Hainfeld, J., & Bittner, J. (1978).
Preliminary measurements of uranium atom motion on carbon films at low temperatures.
Ultramicroscopy 3, 81-86.

7. Lipka, J. J., Lippard, S. J., & Wall, J. S. (1979).
Visualization of polymercurimethane labeled fd bacteriophage in the STEM.
Science 206, 1419-1421.

8. Wall, J. S., Hainfeld, J. F., Bartlett, P. A., & Singer, S. J. (1982).
Observation of an undecagold cluster compound in the scanning transmission electron microscope.
Ultramicroscopy 8, 397-402.

9. Safer, D., Hainfeld, J. F., Wall, J. S., & Riordan, J. (1982).
Biospecific labeling with undecagold: visualization of the biotin binding sites on avidin.
Science 218, 290-291.

10. Milligan, R. A., Whittaker, M., & Safer, D. (1990).
Molecular structure of F-actin and location of surface binding sites.
Nature 348, 217-221.

11. CaJacob, C. A., Frey, P. A., Hainfeld, J. F., Wall, J. S., & Yang, H. (1985).
Escherichia coli pyruvate dehydrogenase complex. Particle masses of the complex and component enzymes measured by scanning transmission electron microscopy.
Biochemistry 24, 2425-2431.

12. Yang, Y.-S., Datta, A., Hainfeld, J. F., Furuya, F. R., Wall, J. S., & Frey, P. A. (1994).
Mapping the lipoyl groups of the pyruvate dehydrogenase complex by use of gold cluster-labels and scanning transmission electron microscopy.
Biochemistry 33, 9428-37.

13. Hainfeld, J. F. (1995).
Gold, electron microscopy, and cancer therapy.
Scanning Micros. 9, 239-54.

14. Woodcock, C. L. F., Frado, L.-L. Y., & Wall, J. S. (1979).
Direct molecular weight determination of chromatin particles by STEM.
J. Cell Biol. 83, 156a.

15. Woodcock, C. L. F., Frado, L.L. Y., & Wall, J. S. (1980).
Composition of native and reconstituted chromatin particles: direct mass determination by scanning transmission electron microscopy.
Proc. Natl. Acad. Sci. USA 77, 4818-4822.

16. Woodcock, C. L. F., & Frank, J. (1984).
Nucleosome mass distribution using image averaging.
J. Ultrastr. Res. 89, 295-302.

17. Woodcock, C. L. F., Frado, L.L. Y., & Rattner, J. B. (1984).
The higher order structure of chromatin: evidence for a helical ribbon arrangement.
J. Cell Biol. 99, 42-52.

18. Gerchman, S. E., & Ramakrishnan, V. (1987).
Chromatin higher-order structure studied by neutron scattering and scanning transmission electron microscopy.
Proc. Natl. Acad. Sci. USA 84, 7802-7806.

19. Mosesson, M. W., Hainfeld, J., Haschemeyer, R. H., & Wall, J. S. (1981).
Identification and mass analysis of human fibrinogen molecules and their domains by scanning transmission electron microscopy (STEM).
J. Mol. Biol. 153, 695-718.

20. Mosesson, M. W., Church, W. R., DiOrio, J. P., Krishnaswamy, S., Mann, K. G., Hainfeld, J. F., & Wall, J. S. (1990).
Structural model of factors V and Va based on scanning transmission electron microscope (STEM) images and STEM mass analysis.
J. Biol. Chem. 265, 8863-8868.

21. Mosesson, M. W., Fass, D. N., Lollar, P., DiOrio, J. P., Knutson, G. J., Hainfeld, J. F., & Wall, J. S. (1990).
Structural model of porcine factors VIII and VIIIa based on scanning transmission electron microscope (STEM) images and STEM mass analysis.
J. Clin. Invest. 85, 1983-1990.

22. Mosesson, M. W., Nesheim, M. E., DiOrio, J., Hainfeld, J. F., Wall, J. S., & Mann, K. G. (1985).
Studies on the structure of bovine factor V by scanning transmission electron microscopy (STEM).
Blood 65, 1158-1162.

23. Mosesson, M. W., DiOrio, J. P., Siebenlist, K. R., Wall, J. S., & Hainfeld, J. F. (1993).
Evidence for a second type of fibril branch point in fibrin polymer networks, the trimolecular branch junction.
Blood 82, 1517-1521.

24. Hough, P. V. C., Mastrangelo, I. A., Wall, J. S., Hainfeld, J. F., Simon, M. N., & Manley, J. L. (1982).
DNA-protein complexes spread on N2-discharged carbon film and characterized by molecular weight and its projected distribution. Nucleosome formation in an in vitro transcription system.
J. Mol. Biol. 160, 375-386.

25. Mastrangelo, I. A., Hough, P. V. C., Wilson, V. G., Wall, J. S., Hainfeld, J. F., & Tegtmeyer, P. (1985).
Monomers through trimers of large tumor antigen bind in region I and monomers through tetramers bind in region II of simian virus 40 origin of replication DNA as stable structures in solution.
Proc. Natl. Acad. Sci. USA 82, 3626-3630.

26. Ryder, K., Vakalopoulou, E., Mertz, R., Mastrangelo, I., Hough, P., Tegtmeyer, P., & Fanning, E. (1985).
Seventeen base pairs of region I encode a novel tripartite binding signal for SV40 T antigen.
Cell 42, 539-548.

27. Mastrangelo, I. A., Hough, P. V. C., Wall, J. S., Dodson, M., Dean, F. B., & Hurwitz, J. (1989).
ATP-dependent assembly of double hexamers of SV40 T antigen at the viral origin of DNA replication.
Nature 338, 658-662.

28. Hough, P. V. C., Mastrangelo, I. A., Wall, J. S., Hainfeld, J. F., Sawadogo, M., & Roeder, R. G. (1987).
The gene-specific initiation factor USF (upstream stimulatory factor) bound at the adenovirus type 2 major late promoter: mass and three-dimensional structure.
Proc. Natl. Acad. Sci. USA 84, 4826-4830.

29. Mastrangelo, I. A., Courey, A. J., Wall, J. S., Jackson, S. P., & C., H. P. V. (1991).
DNA looping and Sp1 multimer links: a mechanism for transcriptional synergism and enhancement.
Proc. Natl. Acad. Sci. USA 88, 5670-5674.

30. Stasiak, A., Tsaneva, I. R., West, S. C., Benson, C. J. B., Yu, X., & Egelman, E. H. (1994).
The Eschericia coli RuvB branch migration protein forms double hexameric rings around DNA.
Proc. Natl. Acad. Sci. USA 91, 7618-7622.

31. Turnquist, S., Simon, M., Egelman, E., & Anderson, D. (1992).
Supercoiled DNA wraps around the bacteriophage Phi-29 head-tail connector.
Proc. Natl. Acad. Sci. USA 89, 10479-10483.

32. Yu, X., & Egelman, E. H. (1992).
Structural data suggest that the active and inactive forms of Rec A filament are not simply interconvertible.
J. Mol. Biol. 227, 334-346.

33. Yu, X., & Egelman, E. H. (1993).
The LexA repressor binds within the deep helical groove of the activated RecA filament.
J. Mol. Biol. 231, 29-40.

34. Steven, A. C., Wall, J., Hainfeld, J., & Steinert, P. M. (1982).
The structure of fibroblastic intermediate filaments: analysis by scanning transmission electron microscopy.
Proc. Natl. Acad. Sci. USA 79, 3101-3105.

35. Steven, A. C., Hainfeld, J. F., Trus, B. L., Wall, J. S., & Steinert, P. M. (1983).
Epidermal keratin filaments assembled in vitro have masses-per-unit-length that scale according to average subunit mass: structural basis for homologous packing of subunits in intermediate filaments.
J. Cell Biol. 97, 1939-1944.

36. Steven, A. C., Hainfeld, J. F., Trus, B. L., Wall, J. S., & Steinert, P. M. (1983).
The distribution of mass in heteropolymer intermediate filaments assembled in vitro: STEM analysis of vimentin/desmin and bovine epidermal keratin.
J. Biol. Chem. 258, 8323-8329.

37. Steinert, P. M., Steven, A. C., & Roop, D. R. (1985).
The molecular biology of intermediate filaments.
Cell 42, 411-419.

38. Steven, A. C., Trus, B. L., Hainfeld, J. F., Wall, J. S., & Steinert, P. M. (1985).
Conformity and diversity in the structures of intermediate filaments.
Ann. NYAS 455, 371-380.

39. Steven, A. C., Hainfeld, J. F., Trus, B. L., Steinert, P. M., & Wall, J. S. (1984).
Radial distributions of density within macromolecular complexes determined from dark-field electron micrographs.
Proc. Natl. Acad. Sci. USA 81, 6363-6367.

40. Johnson, K. A., & Wall, J. S. (1983).
Structure and molecular weight of the dynein ATPase.
J. Cell Biol. 96, 669-678.

41. Witman, G. B., Johnson, K. A., Pfister, K. K., & S., W. J. (1983).
Fine structure and molecular weight of the outer arm dyneins of chlamydomonas.
J. Submicros. Cytol. 15, 193-197.

42. Marchese-Ragona, S., Wall, J. S., & Johnson, K. (1988).
Structure and mass analysis of 14S dynein obtained from Tetrahymena cilia.
J. Cell Biol. 106, 127-132.

43. Hastie, A. T., Marchese-Ragona, S. P., Johnson, K. A., & Wall, J. S. (1988).
The structure and mass of mammalian respiratory ciliary outer arm 19S dynein.
Cell Motil. Cytoskeleton 11, 157-166.

44. Mandiyan, V., Tumminia, S., Wall, J. S., Hainfeld, J. F., & Boublik, M. (1991).
Assembly of the Escherichia coli 30S ribosomal subunit reveals protein dependent folding of the 16S rRNA domains.
Proc. Natl. Acad. Sci. USA 88, 8174-8178.

45. Tumminia, S. J., Hellmann, W., Wall, J. S., & Boublik, M. (1994).
Visualization of the protein-nucleic acid interactions involved in the in vitro assembly of the Eschericia coli 50S ribosomal subunit.
J. Mol. Biol. 235, 1239-1250.

46. Liu, D. J., & Day, L. A. (1994).
Pf1 virus structure: helical coat protein and DNA with paraxial phosphates.
Science 265(July 29), 671-674.

47. Marzec, C. J., & Day, L. A. (1994).
An electrostatic spatial resonance model for coaxial helical structures with applications to the filamentous bacteriophages.
Biophysical J. 67, 2205-2222.

48. Paredes, A. M., Simon, M. N., & Brown, D. T. (1992).
The mass of the sindbis virus nucleocapsid suggests it has t=4 icosahedral symmetry.
Virology 187, 329-332.

49. Steven, A. C., Trus, B. L., Maizel, J. B., Unser, M., Parry, D. A. D., Wall, J. S., Hainfeld, J. F., & Studier, F. W. (1988).
The molecular structure of a viral receptor-recognition protein - the GP17 tail-fiber of bacteriophage T7.
J. Mol. Biol. 200, 351-365.

50. Kapp, O. H., Qabar, A. N., Bonner, M. C., Stern, M. S., Walz, D. A., Schmuck, M., Pilz, I., Wall, J. S., & Vinogradov, S. (1990).
Quaternary structure of the giant extracellular haemoglobin of the leech Macrobdella decora.
J. Mol. Biol. 213, 141-158.

51. Hamilton, M. G., Herskovits, T. H., Furcinitti, P. S., & Wall, J. S. (1989).
Scanning transmission electron microscopic study of molluscan hemocyanins in various aggregation states. Comparison with light scattering molecular weights.
J. Ultrastr. & Mol. Str. Res. 102, 221-228.

52. Trent, J. D., Nimmesgern, E., Wall, J. S., Hartl, F.U., & Horwich, A. L. (1991).
A molecular chaperone from a thermophilic archaebacterium is related to a eukaryotic protein t complex polypeptide-1.
Nature 354, 490-493.

53. Braig, K., Simon, M., Furuya, F., Hainfeld, J. F., & Horwich, A. L. (1993).
A polypeptide bound by the chaperonin groEL is localized within a central cavity.
Proc. Natl. Acad. Sci. USA 90, 3978-3982.

54. Cheng, R. H., Caston, J. R., Wang, G.J., Gu, F., Smith, T. J., Baker, T. S., Bozarth, R. F., Trus, B. L., Cheng, N., Wickner, R. B., & Steven, A. (1994).
Fungal virus capsids are cytoplasmic compartments for the replication of double-stranded RNA, formed as icosahedral shells of asymetric Gag dimers.
J. Mol. Biol. 244, 255-258.

55. Hainfeld, J. F., & Furuya, F. F. (1994).
Silver enhancement of Nanogold and undecagold.
In M. A. Hayat (Eds.), Immunigold silver staining: Methods and applications San Diego, CA: Academic Press.

56. Hainfeld, J. F., Safer, D., Wall, J. S., Simon, M., Lin, B., & Powell, R. D. (1994).
Methylamine vanadate (nanovan) negative stain.
In G. W. Bailey (Ed.), 52nd Ann. Microscopy Society of America, (pp. 132-133). New Orleans: San Francisco Press.

57. Ksiezak-Reding, H., & Wall, J. S. (1994).
Mass and physical dimensions vary in two distinct populations of paired helical filaments in Alzheimer disease.
Neurobiology of Aging 15, 11-19.

58. Makhov, A. M., Hannah, J. H., Brennan, M. J., Trus, B. L., E., K., Conway, J. F., Wingfield, P. T., Simon, M. N., & Steven, A. C. (1994).
Filamentous hemagglutinin of Bordetella pertussis: A bacterial adhesin formed as a 50-nm monomeric rigid rod based on a 19-residue repeat motif rich in beta strands and turns.
J. Mol. Biol. 241, 110-124.

59. Milne, J. C., Furlong, D., Hanna, P. C., Wall, J. S., & Collier, R. J. (1994).
Anthrax protective toxin forms oligomers during intoxication of mammalian cells.
J. Biol. Chem. 269, 20606-20612.

60. Norcum, M. T., Wilkinson, D. A., Carlson, M. C., Hainfeld, J. F., & Carlson, G. M. (1994).
Structure of phosphorylase kinase.
J. Mol. Biol. 241, 94-102.

61. Parsell, D. A., Kowal, A. S., & Lindquist, S. (1994).
The S. cerevisiae Hsp 104 protein: Purification and characterization of ATP-induced structural changes.
J. Biol. Chem. 269, 4480-4487.

62. Stasiak, A., Tsaneva, I. R., West, S. C., Benson, C. J. B., Yu, X., & Egelman, E. H. (1994).
The Eschericia coli RuvB branch migration protein forms double hexameric rings around DNA.
Proc. Natl. Acad. Sci. USA 91, 7618-7622.

63. Stenger, J. E., Tegtmeyer, P., Mayr, G. A., Reed, M., Wang, Y., Wang, P., Hough, P. V. C., & Mastrangelo, I. A. (1994).
p53 oligomerization and DNA looping are linked with transcriptional activation.
EMBO J. 13(24), 6011-6020.

64. Thomas, D., Schultz, P., Steven, A. C., & Wall, J. S. (1994).
Mass analysis of biological macromolecular complexes by STEM.
Biol. Cell 80, 181-192.

65. Tumminia, S. J., Hellmann, W., Wall, J. S., & Boublik, M. (1994).
Visualization of the protein-nucleic acid interactions involved in the in vitro assembly of the Eschericia coli 50S ribosomal subunit.
J. Mol. Biol. 235, 1239-1250.

66. Wilkinson, D. A., Marion, T. N., Tillman, D. M., Norcum, M. T., Hainfeld, J. F., Seyer, J. M., & Carlson, G. M. (1994).
An epitope proximal to the carboxyl terminus of the ?-subunit is located near the lobe tips of the phosphorylase kinase hexadecamer.
J. Mol. Biol. 235, 974-982.

68. Flanagan, J. M., Wall, J. S., Capel, M. s., Schneider, D. K., & Shanklin, J. (1995).
Scanning transmission electron microscopy and small-angle scattering provide evidence that native Escherichia coli clpP is a tetradecamer with an axial pore.
Biochemistry 34, 10910-10917.

69. Gong, X., Radebaugh, C. A., Geiss, G. K., Simon, M. N., & Paule, M. R. (1995).
Site directed photo-cross-linking of rRNA transcription initiation complexes.
Mol. Cell. Biol. 15, 4956-4963.

70. Hainfeld, J. F. (1995).
Gold, electron microscopy, and cancer therapy.
Scanning Micros. 9, 239-256.

71. Marchese-Ragona, S. P., & Wall, J. S. (1995).
Scanning-transmission electron-microscopic analysis of the isolated dynein ATPase.
Methods in Cell Biology 47, 171-175.

72. Mosesson, M. W., Siebenlist, K. R., DiOrio, J. P., Hainfeld, J. F., & Wall, J. S. (1995).
The role of fibrinogen-D domain intermolecular association sites in the polymerization of fibrin and fibrinogen Tokyo-II (gamma 275 Arg->Cys).
J. Clin Inv. 96, 1053-1058.

73. Mosesson, M. W., Siebenlist, K. R., Hainfeld, J. F., & Wall, J. S. (1995).
The covalent structure of factor-XIIIA-cross-linked fibrinogen fibrils.
J. Struct. Biol. 115, 88-101.

74. Nojima, D., Linck, R. W., & Egelman, E. H. (1995).
At least one of the protofilaments in flagellar microtubules is not composed of tubulin.
Current Biology 5, 158-167.

75. Siebenlist, K. R., Meh, D. A., Wall, J. S., Hainfeld, J. F., & Mosesson, M. W. (1995).
Orientation of carboxy-terminal regions of fibrin gamma chain dimers determined from the crosslinked products formed in mixtures of fibrin, fragment D and factor XIIIa.
Thrombosis & Haemostasis 74, 1113-1119.

76. Sosinsky, G. (1995).
Mixing of connexins in gap junction membrane channels.
Proc. Natl. Acad. Sci. USA 92, 9210-9214.

77. Wingfield, P. T., Stahl, S. J., Williams, R. W., & Steven, A. C. (1995).
Hepatitis core antigen produced in Escherichia coli: subunit composition, conformational analysis, and in vitro capsid assembly.
Biochemistry 34, 4919-4932.

78. Blackwell, L. J., Borowiec, J. A., & Mastrangelo, I. A. (1996).
Single-stranded binding alters human replication protein A structure and facilitates interaction with DNA-dependent protein kinase.
Mol. Cell Biol. 16, 4798-4807.

79. Cerritelli, M. E., & Studier, F. W. (1996).
Assembly of T7 capsids from independently expressed and purified head protein and scaffolding protein.
J. Mol. Biol. 258, 286-298.

80. Cerritelli, M. E., & Studier, F. W. (1996).
Purification and characterization of T7 head-tail connectors expressed from the cloned gene.
J. Mol. Biol. 258, 299-307.

81. Cerritelli, M. E., Wall, J. S., Simon, M. N., Conway, J. F., & Steven, A. C. (1996).
Stoichiometry and domainal organization of the long tailfiber of bacteriophage T4: a hinged viral adhesin.
J. Mol. Biol. 260, 767-780.

82. Citovsky, V., Guralnick, B., Simon, M. N., & Wall, J. S. (1997).
The molecular structure of agrobacterium VirE2-single stranded DNA complexes involved in nuclear import.
J. Mol. Biol. 271, 718-27.

83. Gregori, L., Hainfeld, J. F., Simon, M. N., & Goldgaber, D. (1997).
Binding of Amyloid Beta Protein to the 20 S Proteasome.
J. Biol. Chem. 272, 58-62.

84. Ksiezak-Reding, H., Tracz, E., Yang, L., Dickson, D. W., Simon, M., & Wall, J. S. (1996).
Ultrastructural instability of paired helical filaments from corticobasal degeneration as examined by scanning transmission electron microscopy (STEM).
Am. J. Path. 149(N2), 639-651.

85. Lamy, J. N., Green, B. N., Toulmond, A., Wall, J. S., Weber, R. E., & Vinogradov, S. N. (1996).
The giant hexagonal bilayer hemoglobins.
Chemical Reviews 96(N8), 3113-3124.

86. Lin, H., Simon, M. N., & Black, L. (1997).
Purification and characterization of the small subunit of phage T4 terminase, gp16, required for DNA packaging.
J. Biol. Chem. 272, 3495-3501.

87. Martin, P. D., Kuchomov, A. R., Green, B. N., Oliver, R. W. A., Braswell, E. H., Wall, J. S., & Vinogradov, S. N. (1996).
Mass spectrometric composition and molecular mass of Lumbricus terrestris hemoglobin: a refined model of its quaternary structure.
J. Mol. Biol. 255, 154-169.

88. Mosesson, M. W., Siebenlist, K. R., Hainfeld, J. F., Wall, J. S., Soria, J., Soria, C., & Caen, J. P. (1996).
The relationship between the fibrinogen D domain selfassociation/cross-linking site (gamma-xl) and the fibrinogen Dusart abnormality (A-alpha R554C-albumen) - clues to thrombophilia in the Dusart-syndrom.
J. Clin. Invest. 97(N10), 2342-2350.

89. Sharma, P. K., Kuchumov, A. R., Chottard, G., Martin, P., Wall, J. S., & Vinogradov, S. N. (1996).
The role of the dodecamer subunit in the dissociation and reassembly of the hexagonal bilayer structure of Lumbricus terrestris hemoglobin.
J. Biol. Chem. 271, 8754-8762.

90. Yu, X., Jezewska, M. J., Bujalowski, W., & Egelman, E. H. (1996).
The E. coli DnaB helicase can exit in different quaternary states.
J. Mol. Biol. 259, 7-14.

91. Zal, F., Lallier, F. H., Wall, J. S., Vinogradov, S. M., & Toulmond, A. (1996).
The multi-hemoglobin system of the hydrothermal vent tube worm Riftia pachyptila. I. Reexamination of the number and masses of its constituents.
J. Biol. Chem. 271, 8869-88

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