Molecular characteristics and differential expression of two nuclear factors containing the FHA domain in Arabidopsis

The forkhead-associated (FHA) domain identified in a wide variety of proteins is a small module that recognizes phosphothreonine epitopes on proteins. AtFHAq and AtFHA2 of Arabidopsis are homologs of tobacco NtFHA1 encoding a FHA domain-containing transcription activator. Previously, we showed that the FHA domain of NtFHA1 functionally substitutes for that of the Fhl1 forkhead transcription factor involved in rRNA processing in yeast, despite significant differences in their protein structures. In this study, we characterized AtFHA1 and AtFHA2 of Arabidopsis and analyzed their expression patterns. AtFHA1 and AtFHA2 contain an N-terminal FHA domain and a C-terminal acidic region, but lack any known DNA-binding motifs. Both proteins were targeted to the nucleus. GUS staining of transgenic Arabidopsis plants containing the promoter-GUS fusion gene demonstrated high expression of AtFHA2 in whole tissues throughout plant development. In contrast, AtFHA1 was temporally expressed at low levels in roots and vascular tissues of stems. Interestingly, AtFHA1 expression was significantly induced in whole plants in response to external stimuli. The observed differential expression of AtFHA1 and AtFHA2 implies that these two genes have distinct regulatory roles in Arabidopsis.