THE GENETIC REQUIREMENTS FOR UMUDC-MEDIATED COLD SENSITIVITY ARE DISTINCT FROM THOSE FOR SOS MUTAGENESIS

The umuDC operon of Escherichia coli, a member of the SOS regulon, is required for SOS mutagenesis. Following the posttranslational processi ng of UmuD to UmuD' by RecA-mediated cleavage, UmuD' acts in concert w ith UmuC, RecA, and DNA polymerase III to facilitate the process of tr anslesion synthesis, which results in the introduction of mutations. C onstitutive expression of the umuDC operon causes an inhibition of gro wth at 30 degrees C (cold sensitivity). The umuDC-dependent physiologi cal phenomenon manifested as cold-sensitive growth is shown to differ from SOS mutagenesis in two respects, Intact UmuD, the form inactive i n SOS mutagenesis, confers a significantly higher degree of cold sensi tivity in combination with UmuC than does UmuD'. In addition, umuDC-me diated cold sensitivity, unlike SOS mutagenesis, does not require recA function. Since the RecA protein mediates the autodigestion of UmuD t o UmuD', this finding supports the conclusion that intact UmuD is capa ble of conferring cold sensitivity in the presence of UmuC, The degree of inhibition of growth at 30 degrees C correlates with the levels of UmuD and UmuC, which are the only two SOS-regulated proteins required to observe cold sensitivity. Analysis of the cellular morphology of s trains that exhibit cold sensitivity for growth led to the finding tha t constitutive expression of the umuDC operon causes a novel form of s uLA- and sfiC-independent filamentation at 30 degrees C. This filament ation is observed in a strain constitutively expressing the single, ch romosomal copy of umuDC and can be suppressed by overexpression of the ftsQAZ operon.