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Published online by Cambridge University Press: 14 April 2009
The ftsZ gene of Escherichia coli, which codes for an essential cell division protein, is subjected to multiple regulation, as shown in part with studies using an ftsZ::lacZ operon fusion located on phage λJFLIOO. Using this same fusion, we sought to isolate regulatory mutants overexpressing ftsZ by selecting mutants able to grow on lactose. One Lac+ mutant was obtained which overexpressed the ftsZ::lacZ fusion 70-fold. The mutation responsible for the overexpression lies in a new gene, cot, located near 56 min on the E. coli genetic map. The cot mutation probably affects the transcription of a chromosomal open reading frame, 0RF1, lying downstream of the bioA gene and adjacent to the ftzZ::lacZ fusion of the λJFL100 prophage integrated at attλ. Using an ftsZ84(Ts) strain, in which there was a double selection for overexpression of both ftsZ::lacZ and ftsZ+, no Lac+Tr mutants were obtained from 3·6 × 1010 bacteria; the introduction of a mutL allele, increasing spontaneous base substitution mutation rates 75-fold, did not permit us to isolate such a mutant. We conclude that Lac+ftsZ-constitutive mutations cannot be obtained in λJFL100 lysogens by a single base substitution.