Differential transcription initiation and alternative RNA splicing of Knox7, a class 2 homeobox gene of maize.

Knox7, a class 2 homeobox gene has been characterized in maize. A combination of experimental (3'- and 5'-RACE) and bioinformatics approaches supported the idea that Knox7 would be transcribed into two alternative transcripts by differential initiation of transcription. Sequence differences between alternative transcripts, Knox7L the larger and Knox7S the smaller, were confined to their 5' end regions and exon 1 was only found in Knox7L transcripts. Deduced proteins shared the same homeodomain, while an Ala and Ala/Gly rich domain was found only in KNOX7L protein. We hypothesize that KNOX7L and KNOX7S might regulate (differentially) the expression of the same gene(s) by binding competitively to the same cis-acting element(s). Further expression analysis using RT-PCR to amplify cDNA portions corresponding to ORFs of both Knox7 alternative transcripts showed that seven cDNA clones were probably generated by alternative splicing of Knox7L. Alignment of these sequences showed that they are in frame suggesting the existence of the corresponding proteins. Quantitative RT-PCR experiments indicated that Knox7S and Knox7L were expressed in maize embryos during germination. In the same tissue, expression of Knox7S was stimulated by light and ABA and inhibited by GA, two hormones that control germination process.