THE ESCHERICHIA-COLI DNA POLYMERASE-III HOLOENZYME CONTAINS BOTH PRODUCTS OF THE DNAX GENE, TAU AND GAMMA, BUT ONLY TAU IS ESSENTIAL

The replicative polymerase of Escherichia coli, DNA polymerase III, co nsists of a three-subunit core polymerase plus seven accessory subunit s. Of these seven, tau and gamma are products of one replication gene, dnaX. The shorter gamma is created from within the tau reading frame by a programmed ribosomal -1 frameshift over codons 428 and 429 follow ed by a stop codon in the new frame. Two temperature-sensitive mutatio ns are available in dnaX. The 2016(Ts) mutation altered both tau and g amma by changing codon 118 from glycine to aspartate; the 36(Ts) mutat ion affected the activity only of tau because it altered codon 601 (fr om glutamate to lysine). Evidence which indicates that, of these two p roteins, only the longer tau is essential includes the following. (i) The 36(Ts) mutation is a temperature-sensitive lethal allele, and over production of wild-type gamma cannot restore its growth. (ii) An allel e which produced tau only could be substituted for the wild-type chrom osomal gene, but a gamma-only allele could not substitute for the wild -type dnaX in the haploid state. Thus, the shorter subunit gamma is no t essential, suggesting that tau can substitute for the usual function (s) of gamma. Consistent with these results, we found that a functiona l polymerase was assembled from nine pure subunits in the absence of t he gamma subunit. However, the possibility that, in cells growing with out gamma, proteolysis of tau to form a gamma-like product in amounts below the Western blot (immunoblot) sensitivity level cannot be exclud ed.