ABSTRACT
Fragments produced by partial digestion of plastid DNA fromZea mays withEco RI were cloned in Charon 4A. A circular, fine structure physical map of the plastid DNA was then constructed from restriction endonucleaseSal I,Pst I,Eco RI, andBam HI recognition site maps of cloned overlapping segments of the plastid genome. These fragments were assigned molecular weights by reference to size markers from both pBR322 and lambda phage DNA. Because of the detail and extent of the derived map, it has been possible to construct a coordinate system which has a unique zero point and within which all the restriction fragments and previously described structural features can be mapped. A computer program was constructed which will display in a circular fashion any of the above features using an X-Y plotter.
ABSTRACT
Using a gentle method to prepare complexes of duplex DNA and the recBC enzyme for electron microscopy, structures not seen previously were observed to be associated with the double strand DNA exonuclease activity of the enzyme. These were terminal forms and loop + tail(s) structures. Both individual terminal single-stranded tails and single-stranded regions within duplexes were also observed. Observation of the terminal single-stranded structures present after various reaction conditions and after various potentially disruptive treatments helped to identify those structures which might represent true reaction intermediates. Based on these results and those of previous studies, a modified model for the mechanism of recBC enzyme action on double-stranded, linear DNA is proposed.
