Two bZIP proteins from Antirrhinum flowers preferentially bind a hybrid C-box/G-box motif and help to define a new sub-family of bZIP transcription factors

Abstract

Two genes encoding bZIP proteins are expressed in flowers of Antirrhinum majus, predominantly in vascular tissues, carpels and anthers. The sequences of cDNA clones encoding these proteins show them to belong to a distinct sub-family of bZIP proteins which also includes LIP19 from rice and MLIPI5 and OBF1 from maize. The sub-family is characterized by the inclusion of very small proteins consisting of essentially a basic domain and a long leucine zipper. Members also have a conserved upstream open reading frame (uORF) in their 5' leader sequences, implying a common mode of post-transcriptional control. In vitro, the Antirrinum bZIP proteins preferentially bind to a novel hybrid C-box/G-box motif which is found in the promoters of some plant histone genes and of some nuclear-encoded genes with plastidial protein products. Expression of the bZIP proteins in transgenic tobacco under control of the CaMV 35S promoter supports the view that they can regulate expression of genes which contain the preferred target motif within their regulatory sequences, although both enhancement and repression of transcript levels of target genes were observed, indicating that the bZIP proteins probably interact with other factors to modulate transcription in different ways, as has been observed for the small MAF family of bZIP proteins in vertebrates.