Structure and coding properties of a dominant Escherichia coli mutator gene, mutD

The region of the Escherichia coli chromosome coding for the mutD gene was cloned, mutD5 function resides on a 1.2-kilobase fragment coding for a 28-kilodalton (kDa) protein. A deletion end-point analysis shows that the presence of the 28-kDa protein is required for mutD5 function and suggests that the mutD functional region has sufficient capacity to code for a second polypeptide of approximately 20 kDa. Plasmids carrying the mutD5 and mut+ alleles both produce the 28-kDa species. The product of mutD5 is dominant when carried by single and multicopy plasmids. The product of mut+ is dominant only on multicopy plasmids. Thus, mutD5 exhibits negative complementation. We suggest that the 28-kDa protein participates in a multimeric structure, perhaps at the replication fork.