Unstable mutations caused by regional tandem multiplications in the gene for ribosomal protein S4 show thermosensitivity in Escherichia coli.

The nucleotide sequence of the gene rpsD for the ribosomal protein S4 of three thermosensitive mutants of Escherichia coli K12 was determined. It was found that two of them contained regional multiplications of a nucleotide sequence within the gene rpsD. In one case, it is a duplication of a 31 nucleotide stretch and in another it is a triplication of a 41 nucleotide stretch. The thermosensitive phenotype of the two mutants is unstable and reverts at the frequency of approximately 10(-4). The revertants regain the wild-type nucleotide sequence. We postulate that the two mutant genes that contain regional multiplications possibly take an intra-strands secondary structure, which is cleaved to regenerate the wild-type sequence, probably during DNA replication.