The Relationship Between the Gamma and Tau subunits of DNA Polymerase III and RecR

Here is the cluster under discussion. There are three genes in the heart of the cluster:
  1. dnaX: DNA polymerase III subunits gamma and tau (EC 2.7.7.7)
  2. ybaB: Transcriptional regulatory protein
  3. recR: Recombination protein RecR
The ybaB gene might well be more properly annotated as "hypothetical protein" or "gene of unknown function".
The situation in E.coli has been described in Transcriptional organization of the Escherichia coli dnaX gene, by Flower and McHenry. Here is what they say in the abstract:
We have determined the transcriptional organization of the Escherichia coli dnaX gene, the structural gene for both the gamma and tau subunits of DNA polymerase III holoenzyme. By S1 nuclease protection and primer extension mapping of transcripts encoding the dnaX products, one primary promoter of dnaX has been identified that initiates transcription 37 nucleotides upstream from the first codon. dnaX resides in an operon with two recently sequenced genes, orf12, encoding an unidentified product, and recR, the structural gene for a protein involved in the recF pathway of recombination. Under conditions of balanced growth, a very small amount of transcription from the upstream apt promoter (less than 5%) contributes to the expression of tau and gamma, too low for apt to be considered to be on an operon with dnaX, orf12, and recR are transcribed from an independent promoter as well as from the dnaX promoter, providing a mechanism for orf12 and recR to be regulated independent of dnaX. Transcription of the dnaX-orf12-recR operon is terminated upstream from the previously characterized heat shock gene htpG. The dnaX and orf12-recR promoters, cloned into a promoter detection vector, efficiently direct the expression of the downstream reporter gene, lacZ. These results extend our knowledge of the genetic and transcriptional organization of this region of the E. coli chromosome. The transcriptional organization has been defined as follows: apt, dnaX-orf12-recR, htpG. All of these genes are transcribed in the clockwise direction and only dnaX, orf12 and recR are contained in the dnaX operon.

The three-gene gene cluster is conserved throughout numerous phylogenetically diverse genomes, as is apparent from the graphic display of the pinned regions.
The dnaX gene, itself, is pretty interesting. In The gamma subunit of DNA polymerase III holoenzyme of Escherichia coli is produced by ribosomal frameshifting Flower and McHenry descibe how the cell produces individual subunits via 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%.˜
McInerney and O'donnell discuss the role of RecR in replication restart in Replisome Fate upon Encountering a Leading Strand Block and Clearance from DNA by Recombination Proteins. They stated
Genetic studies have identified at least four gene products required for replication restart, RecF, RecO, RecR, and RecA. We find here that these proteins displace a stalled polymerase at a DNA template lesion.

So, we have two subunits of the DNA polymerase III strongly connected to a component of replication recovery and a gene of unknown function.