In a previous paper [Berger et al., PNAS 91 7732, 1994], a theory of virus shell formation was proposed in which shell assembly is directed by local interactions of the coat and scaffolding subunits. This theory requires that the same chemical subunits assume different, stable conformations depending on their position in the shell. During assembly, the conformation of a protein subunit dictates the conformations of its neighboring subunits. It was shown that these local interactions could be designed so as to generate shells that have the same geometric structure as virus capsids. Different sets of local interactions, or local rules, were designed to produce different final shell geometries. In this paper, local rules are given that assemble a T = 7 shell such that a small change in these rules produces a T = 4 shell. This is intriguing since evidence has been accumulating that some T = 7 shells are closely related to T = 4 shells. These local rules also predict that hexamers in the assembled procapsid would have approximate two-fold rotational symmetry. This symmetry is exemplified by the elongation of hexamers observed in many T = 7 viruses. These rules also provide a possible explanation for spiraling and tubular malformations.

Carotovoricin Er is a phage-tail-like bacteriocin produced by Erwinia carotovora subsp. carotovora strain Er, a causative agent for soft rot disease in plants. Here we studied binding and killing spectra of carotovoricin Er preparations for various strains of the bacterium (strains 645Ar, EC-2, N786, and P7) and found that the preparations contain two types of carotovoricin Er with different host specificities; carotovoricin Era possessing a tail fiber protein of 68 kDa killed strains 645Ar and EC-2, while carotovoricin Erb with a tail fiber protein of 76 kDa killed strains N786 and P7. The tail fiber proteins of 68 and 76 kDa had identical N-terminal amino acid sequences for at least 11 residues. A search of the carotovoricin Er region in the chromosome of strain Er indicated the occurrence of a DNA inversion system for the tail fiber protein consisting of (i) two 26-bp inverted repeats inside and downstream of the tail fiber gene that flank a 790-bp fragment and (ii) a putative DNA invertase gene with a 90-bp recombinational enhancer sequence. In fact, when a 1,400-bp region containing the 790-bp fragment was amplified by a PCR using the chromosomal DNA of strain Er as the template, both the forward and the reverse nucleotide sequences of the 790-bp fragment were detected. DNA inversion of the 790-bp fragment also occurred in Escherichia coli DH5 � when two compatible plasmids carrying either the 790-bp fragment or the invertase gene were cotransformed into the bacterium. Furthermore, hybrid carotovoricin CGE possessing the tail fiber protein of 68 or 76 kDa exhibited a host range specificity corresponding