Genetic analysis and phenotypic characterization of Lon mutants of Escherichia coli K-12

A systematic study of a collection of Lon⁻ mutants has been made in order to determine whether their pleiotropic phenotype is due to mutations affecting one or more genes. A fine structure map of the lon locus was constructed by Pl mediated generalized transduction. The lon⁻ mutations were found to map in two "clusters" within the region. Phenotypic characterization of a set of isogenic Lon⁻ strains derived from these experiments indicated that all Lon-associated phenotypes (e.g. sensitivity to UV irradiation, decreased ability to inherit plasmid and prophage, abnormal polypeptide degradation and regulation of capsular polysaccharide biosynthesis) are differentially expressed in Lon⁻ strains. A direct correlation exists between the intracistronic ordering of the lon⁻ alleles and the degree of expression the Lon⁻ phenotypes in each strain.

All isogenic Lon⁻ strains exhibit conditional lethality upon a nutritional shift-up. However, some filamenting Lon⁻ mutants are not able to overcome this defect when exposed to growth conditions known to promote cell division in Lon⁻ strains. Evidence was obtained that suggest a role for nucleotide pools in the control of cell division and capsular polysaccharide production.

Reversion studies indicated that all lon⁻ mutations studied are point mutations. The failure to generate deletions of the lon region in χ573, an F' strain carrying the lac to minE region on the plasmid, and the inability to cure F' strains carrying a lon⁻ mutation on the plasmid suggest that the lon gene product may be indispensable for the cell's survival.

From transductional crosses, two intermediate phenotypic classes: UV-resistant, mucoid (UV^RMuc), (Class A) and UV-sensitive, nonmucoid (UV^SRou) (Class B), were obtained that did not segregate colonies of the opposite morphology. Genetic analysis of these strains by back-transduction into a proC⁻ lon⁺ background, indicated that complete genetic separation of all Lon-associated phenotypes tested was not achieved, although differences in the expression of some of these persisted.

Data obtained from complementation analysis ruled out the presence of two genes at the lon locus. The patterns of complementation observed were compatible with the existence of one lon gene, having at least two distinct domains, and whose product is a multifunctional polypeptide.