Bacteriophage m29 of Bacillus subtilis is an excellent model for studies of viral DNA packaging which is a poorly understood aspect of viral morphogenesis. The in vitro packaging system of m29 DNA is as efficient as in vivo assembly. This permits models of DNA packaging to be tested and the structure of components of the DNA packaging machine to be determined. m29 DNA with covalently bound gene product 3 at each terminus (DNA-gp3) is translocated efficiently in vitro into a preformed protein shell (prohead) by a machine that includes the prohead portal vertex (head-tail connector), the 174-base m29-encoded prohead RNA (pRNA) and the ATPase gp16. The prohead, connector, pRNA and gp16 are all overproduced from cloned genes permitting the structure of these components in sequential interactions with DNA-gp3 to be studied. Supercoiled pBR322 DNA wraps around the outside of the isolated m29 connector. This is hypothesized to reflect the initiation phase of DNA packaging. In this model, proheads should also bind supercoiled DNA and m29 DNA-gp3 should be supercoiled as a prerequisite for packaging. By TEM and the STEM, DNA-gp3 appeared to form lariats with loops of variable size. The addition of gp16 and ATP appeared to coil the loops. Work is in progress to determine the structure of DNA-gp3-gp16-prohead complexes.
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