DNA sequence analysis of spontaneous tonB deletion mutations in a polA1 strain of Escherichia coli K12

By DNA sequence analysis, we have determined a spectrum of 61 spontaneous m utations occurring in the endogenous tonB gene in the polA1 strain of Esche richia coli. The overall mutation frequency was approximately 2.4-fold high er in the polA1 strain and this was attributable to enhanced rates of delet ion and frameshift mutations. Among 39 deletions, a hot spot (17 mutations) was detected: a 13-bp deletion presumably directed by a 3-bp repeated sequ ence at its end points. The remaining 22 were distributed among 19 differen t mutations either flanked (16/19) or not flanked (3/19) by repeated sequen ces. Single-base frameshifts accounted for 8 mutations of either repeated ( 3/8) or nonrepeated (5/8) bases among which 6 were minus one frameshift. In contrast to previous reports, we did not frequently observe a 5'-GTGG-3' s equence in the vicinity of the deletions and frameshifts. The results prese nted here indicated an anti-deletion and anti-frameshift role for DNA polym erase I. (C) 1999 Academic Press.