Eight UCA codons differentially affect the expression of the lacZ gene in the divE42 mutant of Escherichia coli

In the divE mutant, which has a temperature-sensitive mutation in the tRNA( 1)(Ser) gene, the synthesis of beta-galactosidase is dramatically decreased at the non-permissive temperature. In Escherichia coli, the UCA codon is o nly recognized by tRNA(1)(Ser). Several genes containing UCA codons are nor mally expressed at 42degreesC in the divE mutant. Therefore, it is unlikely that the defect is due to the general translational deficiency of the muta nt tRNA(1)(Ser). In this study, we constructed mutant lacZ genes, in which one or several UCA codons at eight positionswere replaced with other serine codons such as UCU or UCC, and we examined the expression of these mutant genes in the divE mutant. We found that a single U CA codon at position 6 o r 462 was sufficient to cause the same level of reduced beta-galactosidase synthesis as that of the wild-type lacZ gene, and that the defect in beta-g alactosidase synthesis was accompanied by a low level of lacZ mRNA. It was also found that introduction of an rne-1 pnp-7 double mutation restored the expression of mutant lacZ genes with only UCA codons at position 6 or 462. A polarity suppressor mutation in the rho gene had no effect on the defect in lacZ gene expression in the divE mutant. We propose a model to explain these results.