Isolation and characterization of AtMLH1, a MutL homologue from Arabidopsis thaliana

DNA mismatch repair systems play an essential role in the maintenance of ge netic information in living organisms and are also implicated in genetic re combination and genome stability. Using degenerate primers, we have cloned the first plant homologue of the E. coli MutL gene, which we have called At MLH1 for Arabidopsis thaliana MutL-homologue 1. AtMLH1 is present as a sing le-copy gene in the Arabidopsis genome and is located on the top arm of chr omosome 4. Sequence analysis revealed that the product of this gene shows e xtensive sequence homology with other eukaryotic MLH1 proteins. As mlh1-def icient lines would be useful for studying the biological function of this g ene, several populations that had been mutagenized using T-DNA and transpos on insertions were screened to identify such mutants. One line that carries a T-DNA insertion in the promoter region of the AtMLH1 gene was isolated. Surprisingly, although the insertion occurred only approximate to 80 bp ups tream of the putative transcription start site, Northern analyses revealed very low but similar amounts of AtMLH1 transcript in both the wild type and the T-DNA insertion lines. RT-PCR analyses suggest, however, that transcri ption is initiated further upstream in the insertion line and that the T-DN A may supply this novel initiation site. Finally, no increase in microsatel lite instability - a phenotype often associated with mutations in mismatch repair genes - was observed in plants homozygous for this insertion.