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Conclusive evidence in presented to show that the gene uvrA is dispensable, but the uvrB is indispensable for an Escherichia coli strain carrying gene polA1. We constructed strains E139 (sup-126 polAl uvrB59) and E159 (sup-126 polAl uvrA43) where mutations polAl, uvrB59 and uvrA43 are amber mutations and mutation sup-126 is an amber suppressor mutation effective at 30 degrees C but not at 42 degrees C. At 42 degrees C, strain E139 is inviable but strain E159 viable whereas both are viable at 30 degrees C. Revertants of E139 viable at 42 degrees C occurred spontaneously at a frequency of about 3 X 10(-4). One of the revertants was shown to be caused by suppressor mutation, designated spu, rather than back mutation of the gene uvrB59 or polAl or amber suppressor mutation. Viabilities of the revertants varied from 10(-3) to 1.0 at 42 degrees C compared with those of 30 degrees C. At 42 degrees C, all the revertants with normal viabilities at 42 degrees C were non-filamentous in contrast to the filamentous character of E139. However, strain E159 was viable at 42 degrees C despite its filamentous character. We conclude that the gene uvrB is involved not only in excision repair but also in normal growth in a polA background.

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