Interaction of DNA polymerase III gamma and beta subunits in vivo in Salmonella typhimurium

Suppression of a DnaX temperature-sensitive polymerization defect by mutation in the initiation gene, dnaA, requires functional oriC

Temperature sensitivity of DNA polymerization and growth, resulting from mutation of the tau and gamma subunits of Escherichia coli DNA polymerase III, are suppressed by Cs,Sx mutations of the initiator gene, dnaA. These mutations simultaneously cause defective initiation at 20 degrees C. Efficient suppression, defined as restoration of normal growth rate at 39 degrees C to essentially all the cells, depends on functional oriC. Increasing DnaA activity in a strain capable of suppression, by introducing a copy of the wild-type allele, increasing the suppressor gene dosage or introducing a seqA mutation, reversed the suppression. This suggests that the suppression mechanism depends on reduced activity of DnaACs, Sx. Models that assume that suppression results from an initiation defect or from DnaACs,Sx interaction with polymerization proteins during nascent strand synthesis are proposed.

Mutations in Escherichia coli dnaA which suppress a dnaX(Ts) polymerization mutation and are dominant when located in the chromosomal allele and recessive on plasmids

Extragenic suppressor mutations which had the ability to suppress a dnaX2016(Ts) DNA polymerization defect and which concomitantly caused cold sensitivity have been characterized within the dnaA initiation gene. When these alleles (designated Cs, Sx) were moved into dnaX+ strains, the new mutants became cold sensitive and phenotypically were initiation defective at 20 degrees C (J.R. Walker, J.A. Ramsey, and W.G. Haldenwang, Proc. Natl. Acad. Sci. USA 79:3340-3344, 1982). Detailed localization by marker rescue and DNA sequencing are reported here. One mutation changed codon 213 from Ala to Asp, the second changed Arg-432 to Leu, and the third changed codon 435 from Thr to Lys. It is striking that two of the three spontaneous mutations occurred in codons 432 and 435; these codons are within a very highly conserved, 12-residue region (K. Skarstad and E. Boye, Biochim. Biophys. Acta 1217:111-130, 1994; W. Messer and C. Weigel, submitted for publication) which must be critical for one of the DnaA activities. The dominance of wild-type and mutant alleles in both initiation and suppression activities was studied. First, in initiation function, the wild-type allele was dominant over the Cs, Sx alleles, and this dominance was independent of location. That is, the dnaA+ allele restored growth to dnaA (Cs, Sx) strains at 20 degrees C independently of which allele was present on the plasmid. The dnaA (Cs, Sx) alleles provided initiator function at 39 degrees C and were dominant in a dnaA(Ts) host at that temperature. On the other hand, suppression was dominant when the suppressor allele was chromosomal but recessive when it was plasmid borne. Furthermore, suppression was not observed when the suppressor allele was present on a plasmid and the chromosomal dnaA was a null allele. These data suggest that the suppressor allele must be integrated into the chromosome, perhaps at the normal dnaA location. Suppression by dnaA (Cs, Sx) did not require initiation at oriC; it was observed in strains deleted of oriC and which initiated at an integrated plasmid origin.

The gamma subunit of DNA polymerase III holoenzyme of Escherichia coli is produced by ribosomal frameshifting

The tau and gamma subunits of DNA polymerase III holoenzyme are both products of the dnaX gene. Since tau and gamma are required as stoichiometric components of the replicative complex, a mechanism must exist for the cell to coordinate their synthesis and ensure that both subunits are present in an adequate quantity and ratio for assembly. We have proposed that gamma is produced by a translational frameshift event. In this report, we describe the use of dnaX-lacZ fusions in all three reading frames to demonstrate that gamma, the shorter product of dnaX, is generated by ribosomal frameshifting to the -1 reading frame of the mRNA within an oligo(A) sequence that is followed by a sequence predicted to form a stable secondary structure. Immediately after frameshifting a stop codon is encountered, leading to translational termination. Mutagenesis of the oligo(A) sequence abolishes frameshifting, and partial disruption of the predicted distal secondary structure severely impairs the efficiency. Comparison of the expression of lacZ fused to dnaX distal to the site of frameshifting in the -1 and 0 reading frames indicates that the efficiency of frameshifting is approximately 40%.

Genetic reconstitution of the high-affinity L-arabinose transport system

Expression plasmids containing various portions of araFGH operon sequences were assayed for their ability to facilitate the high-affinity L-arabinose transport process in a strain lacking the chromosomal copy of this operon. Accumulation studies demonstrated that the specific induction of all three operon coding sequences was necessary to restore high-affinity L-arabinose transport. Kinetic analysis of this genetically reconstituted transport system indicated that it functions with essentially wild-type parameters. Therefore, L-arabinose-binding protein-mediated transport appears to require only two inducible membrane-associated components (araG and araH) in addition to the binding protein (araF).

Dilute suppressor dsu acts semidominantly to suppress the coat color phenotype of a deletion mutation, dl20J, of the murine dilute locus

The murine dilute suppressor (dsu) gene is the only unlinked trans-acting suppressor identified in mammals. dsu, which was originally reported to be recessive, was recognized by its ability to suppress the coat color phenotype of a retroviral insertion mutation, dv, of the murine dilute (d) locus. This insertion mutation resulted from the integration of an ecotropic murine leukemia virus into noncoding sequences of the dilute gene. Therefore, dsu may act like other allele-specific recessive suppressors identified in Drosophila melanogaster and yeast that suppress mutations induced by retrotransposon insertions. To investigate this possibility, we have examined whether dsu could suppress a spontaneously arising allele of d, dl20J, which is shown here to result from a 3.5-kilobase deletion. These studies indicate that dsu does not function like other eukaryotic suppressor genes that suppress retrotransposon-induced mutations. We also show that dsu is not, as originally reported, a recessive gene but is semidominantly inherited. Collectively, these results allow us to propose a mechanism for the suppressor activity of dsu.

Genetic analysis in Salmonella typhimurium with a small collection of randomly spaced insertions of transposon Tn10 delta 16 delta 17

We report the isolation of a group of 279 Salmonella typhimurium strains carrying randomly spaced insertions of the minitransposon Tn10 delta 16 delta 17 and describe the use of these strains to facilitate genetic analysis. The insertions were isolated initially in individual recombinant lambda clones from a genomic library. Individual insertions were then moved into the S. typhimurium chromosome, where the distribution of insertion sites relative to standard genetic markers was analyzed in a series of transductional crosses. Since a different, randomly chosen clone was used to generate each insertion, the distribution of insertion positions should have been as random as the cloning events leading to the formation of the library. In agreement with this expectation, most S. typhimurium markers tested were cotransducible with one or more of these Tn10 delta 16 delta 17 insertions. We expect that most new mutations will be quickly classified and mapped by determination of the pattern of cotransduction with this set of insertions. This use is illustrated by the analysis of a group of lac operon fusions regulated by anaerobiosis. We also describe several other applications that should make this collection a useful new tool in S. typhimurium genetics.