Pathways of assimilation of [13N]N2 and 13NH4+ by cyanobacteria with and without heterocysts.

Daughter strand gaps are secondary lesions caused by interrupted DNA synthesis in the proximity of UV-induced pyrimidine dimers. The relative roles of DNA recombination and de novo DNA synthesis in filling such gaps have not been clarified, although both are required for complete closure. In this study, the Escherichia coli E486 and E511 dnaE(Ts) mutants, in which DNA polymerase I but not DNA polymerase III is active at 43 degrees C, were examined. Both mutants demonstrated reduced gap closure in comparison with the progenitor strain at the nonpermissive temperature. These results and those of previous studies support the hypothesis that both DNA polymerase I and DNA polymerase III contribute to gap closure, suggesting a cooperative effort in the repair of each gap. Benzoylated, naphthoylated diethylaminoethyl-cellulose chromatography analysis for persistence of single-strand DNA in the absence of DNA polymerase III activity suggested that de novo DNA synthesis initiates the filling of daughter strand gaps.