The Escherichia coli DNA polymerase III holoenzyme 71.1 kDa τ subunit is a 643 amino acid protein encoded by the dnaX gene. This gene also encodes the holoenzyme 56.5 kDa γ subunit. The τ factor (as a τ' -LacZ' fusion protein) has been isolated and shown to be cleaved in vitro to form γ and a 135 kda C-terminal cleavage product. The τ' -LacZ' fusion protein, γ, and the C-terminal cleavage product have been isolated. N-terminal sequencing has demonstrated that τ and γ share the same N-terminal sequences and that τ is proteolytically cleaved in vitro between residues 498 and 499 to form γ. In addition, residues 420-440 were shown to be present in both τ and γ by use of antibody specific for a synthetic peptide corresponding to that sequence. Some mechanism functions in vivo to ensure that τ and γ are synthesized in a ratio of about one-to-one, as shown by radioimmune precipitation of τ and γ from cellular extracts.
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