Plastidic proteins containing motifs of nuclear transcription factors

Plants are constantly exposed to environmental factors, including biotic and abiotic stresses, which may confer serious damage and affect survival. In order to cope with these stresses, plants have evolved a variety of defense systems. Despite intensive surveys, the molecular mechanisms are still not completely understood, and in particular, information on diversification of proteins is limited. In this article, we focus on examples of proteins changing both cellular localization and functions. Experimentally, six such proteins have so far been identified, all of them containing motifs of nuclear transcription factors, but localized in plastids: a 41-kDa protein from Nicotiana tabacum containing a zinc finger motif, shown to be a chloroplast nucleoid DNA binding protein (CND41); a plastid envelope DNA binding protein (PEND) from Pisum sativum possessing a basic domain plus leucine zipper motif; proteins designated as PD1 and PD3 also isolated from P. sativum, both having AT-hook motifs; a protein with a basic helix-loop-helix (bHLH) motif from Arabidopsis thaliana proposed to be a plastid transcription factor, and designated as PTF1; one wound-induced protein from N. tabacum (NtWIN4) shown to be a bHLH protein and exclusively localized into plastids. Judging from their structures, these proteins could have originated from eukaryotic ancestors. At the N-terminus, they possess clustered basic residues, which might constitute the key structure for conversion from nuclear transcription factors to plastid-resident proteins.