Deletion between direct repeats in bacteriophage T7 gene 1.2.

Deletion mutagenesis in bacteriophage T7 was studied with an insertion-reversion assay involving phage containing inserts of foreign DNA that form 10-bp direct repeats. The precise deletion of the insert restores the function of the non-essential gene and is easily assayed by growth on selection strains of E. coli. Similar inserts with unique direct repeats were placed in either gene 1.2 (dGTPase Inhibitor) or gene 1.3 (DNA ligase). The deletion rates of the inserts were quantified with Luria and Delbrück fluctuation tests. Deletion rates were similar for inserts in both genes indicating that the rates of deletion were not unique to either specific site, or the sequence of the direct repeats. Deletion was independent of functional T7 ligase at 37 degrees C, while an increase in the rate of deletion was noted in some ligase-deficient phage at 43 degrees C. The effect of E. coli DNA Polymerase I on deletion rate was tested and found to decrease deletion rate 60% with the polA1 mutation and 90% with the polA546ex mutation.