Suppression of the Escherichia coli ssb-1 mutation by an allele of groEL

A carboxy-terminal deletion impairs the assembly of GroEL and confers a pleiotropic phenotype in Escherichia coli K-12

A series of COOH-terminal deletions of the chaperonin GroEL have been examined for effects in vivo at haploid copy number on the essential requirement of GroEL for cell growth. Strains with a deletion of up to 27 COOH-terminal amino acids were viable, but not viable strain could be isolated with a deletion of 28 or more codons. When substitutions were placed in the COOH-terminal amino acid Val-521 of the 27-amino-acid-deleted (delta 27) mutant, we found variable effect--Trp and Glu led to inviability, whereas Arg and Gly were viable but slow growing. The effects of the Arg substitution plus deletion (V521R delta) were examined in more detail. Whereas the delta 27 mutant with the wild-type residue Val-521 grew as well as a strain with wild-type GroEL, the V521R delta mutant strain (groEL202) exhibited a broad range of phenotypic defects. These include slow growth; filamentous morphology; a defect in plating lambda; absence of activity of expressed human ornithine transcarbamylase, as seen in other GroEL mutants; and several newly observed defects, such as absence of motility, sensitivity to UV light and mitomycin, a defect in one mode of specialized transduction, and inability to grow on rhamnose. Sucrose gradient analysis of extracts from the V521R delta cells showed a substantially reduced level of GroEL sedimenting at the normal 20S position of the assembled tetradecamer and a relatively large amount of more lightly sedimenting subunits. This indicates that the substitution-deletion mutation interferes with oligomeric assembly of GroEL into its functional form. This is discussed in light of the recently determined crystal structure of GroEL.

Interaction of the heat shock protein GroEL of Escherichia coli with single-stranded DNA-binding protein: suppression of ssb-113 by groEL46

Previous studies from our laboratory have shown that an allele of the heat shock protein GroEL (groEL411) is able to specifically suppress some of the physiological defects of the single-stranded DNA-binding protein mutation ssb-1. A search for additional alleles of the groE genes which may act as suppressors for ssb mutations has led to the identification of groEL46 as a specific suppressor of ssb-113. It has very little or no effect on ssb-1 or ssb-3. All of the physiological defects of ssb-113, including temperature-sensitive growth, temperature-sensitive DNA synthesis, sensitivity to UV irradiation, methyl methanesulfonate, and bleomycin, and reduced recombinational capacity, are restored to wild-type levels. The ssb-113 allele, however, is unable to restore sensitivity of groEL46 cells to phage lambda. The mechanism of suppression of ssb-113 by groEL46 appears to differ from that of ssb-1 by groEL411. The data suggest that GroEL may interact with single-stranded DNA-binding protein in more than one domain.

Physiological roles of the DnaK and GroE stress proteins: catalysts of protein folding or macromolecular sponges?

When organisms ranging from microbes to man are subjected to certain environmental stresses a characteristic 'heat shock' response is observed. In Escherichia coli this response is characterized by the induction of several proteins, three of which are the 70 kilodalton product of the dnaK gene, the 60 kilodalton product of the groEL (mopA) gene and the 15 kilodalton product of the groES (mopB) gene. In this review, utilizing enteric bacteria as model organisms, we focus on the role of these proteins within the context provided by well-established functions of other heat shock products. These facts serve as a starting point from which to speculate upon the in vivo role of these proteins during steady-state growth.

The single-stranded DNA-binding protein of Escherichia coli

The single-stranded DNA-binding protein (SSB) of Escherichia coli is involved in all aspects of DNA metabolism: replication, repair, and recombination. In solution, the protein exists as a homotetramer of 18,843-kilodalton subunits. As it binds tightly and cooperatively to single-stranded DNA, it has become a prototypic model protein for studying protein-nucleic acid interactions. The sequences of the gene and protein are known, and the functional domains of subunit interaction, DNA binding, and protein-protein interactions have been probed by structure-function analyses of various mutations. The ssb gene has three promoters, one of which is inducible because it lies only two nucleotides from the LexA-binding site of the adjacent uvrA gene. Induction of the SOS response, however, does not lead to significant increases in SSB levels. The binding protein has several functions in DNA replication, including enhancement of helix destabilization by DNA helicases, prevention of reannealing of the single strands and protection from nuclease digestion, organization and stabilization of replication origins, primosome assembly, priming specificity, enhancement of replication fidelity, enhancement of polymerase processivity, and promotion of polymerase binding to the template. E. coli SSB is required for methyl-directed mismatch repair, induction of the SOS response, and recombinational repair. During recombination, SSB interacts with the RecBCD enzyme to find Chi sites, promotes binding of RecA protein, and promotes strand uptake.

The groES and groEL heat shock gene products of Escherichia coli are essential for bacterial growth at all temperatures

The products of the groES and groEL genes of Escherichia coli, constituting the groE operon, are known to be required for growth at high temperature (42 degrees C) and are members of the heat shock regulon. Using a genetic approach, we examined the requirement for these gene products for bacterial growth at low temperature (17 to 30 degrees C). To do this, we constructed various groES groEL heterodiploid derivative strains. By inactivating one of the groE operons by a polar insertion, it was shown by bacteriophage P1 transduction that at least one of the groE genes was essential for growth at low temperature. Further P1 transduction experiments with strains that were heterodiploid for only one of the groE genes demonstrated that both groE gene products were required for growth at low temperature, which suggested a fundamental role for the groE proteins in E. coli growth and physiology.