Bacteriophage lambda preconnectors: Purification and structure

Abstract

The morphogenesis of bacteriophage λ proheads is under the control of the four phage genes B, C, Nu3 and E, and the two Escherichia coli genes groEL and groES. It has been shown previously that extracts prepared from cells infected with a λC − E − mutant accumulate a gpB polymer, which behaves as a biologically active intermediate in prohead assembly. This gpB activity has been called a preconnector, as it is probably a precursor to the head-tail connector. We now report the partial purification of biologically active preconnectors and the characterization of its structure. In the electron microscope, preconnectors appear as donut-like structures composed of several subunits displaying radial symmetry. Optical filtration of periodic arrays of preconnectors showed that the structure has 12-fold rotational symmetry. Side views of the preconnector reveal that it resembles an asymmetrical dumbell. This information has been used to construct a three-dimensional model of the preconnector. The implications of this structure for prohead shape and function, and for DNA packaging are discussed.