Characterization of the uup Locus and Its Role in Transposon Excisions and Tandem Repeat Deletions in Escherichia coli

Abstract

Null mutations in the Escherichia coli uup locus (at 21.8 min) serve to increase the frequency of RecA-independent precise excision of transposable elements such as Tn10 and to reduce the plaque size of bacteriophage Mu (Uup(-) phenotype). By the combined approaches of physical mapping of the mutations, complementation analyses, and protein overexpression from cloned gene fragments, we have demonstrated in this study that the Uup(-) phenotype is the consequence of the absence of expression of the downstream gene (uup) of a two-gene operon, caused either directly by insertions in uup or indirectly by the polar effect of insertions in the upstream gene (ycbY). The promoter for uup was mapped upstream of ycbY by primer extension analysis on cellular RNA, and assays of reporter gene expression indicated that it is a moderately active, constitutive promoter. The uup mutations were also shown to increase, in a RecA-independent manner, the frequencies of nearly precise excision of Tn10 derivatives and of the deletion of one copy of a chromosomal tandem repeat, suggesting the existence of a shared step or intermediate in the pathways of these latter events and that of precise excision. Finally, we found that mutations that increase the frequency of precise excision of Tn10 are divisible into two categories, depending upon whether they did (uup, ssb, polA, and topA) or did not (mutHLS, dam, and uvrD) also increase precise excision frequency of the mini-Tn10 derivatives. It is suggested that the differential response of mini-Tn10 and Tn10 to the second category of mutations is related to the presence, respectively, of perfect and of imperfect terminal inverted repeats in them.