In the divE mutant, which has a temperature-sensitive mutation in the tRNA1(Ser) gene, the synthesis of beta-galactosidase is dramatically decreased at the non-permissive temperature. In Escherichia coli, the UCA codon is only recognized by tRNA1(Ser). Several genes containing UCA codons are normally expressed at 42 degrees C in the divE mutant. Therefore, it is unlikely that the defect is due to the general translational deficiency of the mutant tRNA1(Ser). In this study, we constructed mutant lacZ genes, in which one or several UCA codons at eight positions were 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 UCA codon at position 6 or 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-galactosidase 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.