Arabidopsis MutS homologs-AtMSH2, AtMSH3, AtMSH6, and a novel AtMSH7-form three distinct protein heterodimers with different specificities for mismatched DNA

Arabidopsis mismatch repair genes predict MutS-like proteins remarkably sim ilar to eukaryotic MutS homologs-MSH2, MSH3, and MSH6, A novel feature in A rabidopsis is the presence of two MSH6-like proteins, designated AtMSH6 and AtMSH7. Combinations of Arabidopsis AtMSH2 with AtMSH3, AtMSH6, or AtMSH7 proteins-products of in vitro transcription and translation-were analyzed f or interactions by analytical gel filtration chromatography, The AtMSH2 pro tein formed heterodimers with AtMSH3, AtMSH6, and AtMSH7, but no single pro teins formed homodimers. The abilities of the various heterodimers to bind to mismatched 51-mer duplexes were measured by electrophoretic mobility-shi ft assays. Similar to the behavior of the corresponding human proteins, AtM SH2.AtMSH3 heterodimers bound "insertion-deletion" DNA with three nucleotid es (+AAG) or one nucleotide (+T) looped out much better than they bound DNA with a base/base mispair (T/G), whereas AtMSH2.AtMSH6 bound the (+T) subst rate strongly, (T/G) well, and (+AAG) no better than it did a (T/A) homodup lex. However, AtMSH2.AtMSH7 showed a different specificity: moderate affini ty for a (T/G) substrate and weak binding of (+T). Thus, AtMSH2.AtMSH7 may be specialized for lesions/base mispairs not tested or for (T/G) mispairs i n special contexts.