A SINGLE RESIDUE SUBSTITUTION CAUSES A SWITCH FROM THE DUAL DNA-BINDING SPECIFICITY OF PLANT TRANSCRIPTION FACTOR MYB.PH3 TO THE ANIMAL C-MYB SPECIFICITY

Transcription factor MYB.Ph3 from Petunia binds to two types of sequen ces, MBSI and MBSII, whereas murine c-MYB only binds to MBSI, and Am30 5 from Antirrhinum only binds to MBSII. DNA binding studies with hybri ds of these proteins pointed to the N-terminal repeat (R2) as the most involved in determining binding to MBSI and/or MBSII, although some i nfluence of the C-terminal repeat (R3) was also evident. Furthermore, a single residue substitution (Leu(71) --> Glu) in MYB.Ph3 changed its specificity to that of c-MYB, and c-MYB with the reciprocal substitut ion (Glu(132) --> Leu) essentially gained the MYB.Ph3 specificity. Mol ecular modeling and DNA binding studies with site-specific MYB.Ph3 mut ants strongly supported the notion that the drastic changes in DNA bin ding specificity caused by the Leu --> Glu substitution reflect the fa ct that certain residues influence this property both directly, throug h base contacts, and indirectly, through interactions with other base- contacting residues, and that a single residue may establish alternati ve base contacts in different targets. Additionally, differential effe cts of mutations at nonbase-contacting residues in MYB.Ph3 and c-MYB w ere observed, reflecting the importance of protein context on DNA bind ing properties of MYB proteins.