Powerful mutator activity of the polA1 mutation within the histidine region of Escherichia coli K-12.

We examined 122 spontaneous histidine auxotrophs accumulated in overnight cultures of polA1 strains of Escherichia coli K-12 at approximate frequencies of 10(-3). One hundred and thirteen appeared to be minus frameshifts, and nine appeared to be deletions. Of the frameshift mutations, 109 affected the hisC gene, and 4 affected genes hisD, hisH, hisA, and hisI. The lack of base substitutions supported the idea that polymerase-defective polA is a minus frameshift- and deletion-type mutator. Contrary to a previous report, we did not observe superior growth of PolA auxotrophs over their prototrophic progenitors (15 auxotrophs tested). We conclude that the polA1 mutation exerts a powerful mutator activity in this specific genetic context.