The isolation and identification of a light-induced protein in alfalfa sprouts and the cloning of its specific promoter.

We used 2D-PAGE to isolate a light-induced protein (AL-A) that is expressed abundantly in light-growth alfalfa sprouts. The seven amino acids of the N-terminal region of the protein were identified, and we searched for the protein in GenBank using the BLAST program. The results of the homology analysis showed that the amino acid sequence of the isolated protein is most similar to one from a pea plastocyanin. To identify the protein, we amplified and sequenced the DNA fragment encoding AL-A from genomic alfalfa DNA. We found that the AL-A gene was highly homologous (90%) to the sequences from the pea plastocyanin via multiple alignments, and the deduced protein precursor was predicted to be chloroplast-specific via the ChloroP computer program. The protein was named alfalfa-plastocyanin (AL-P). It was characterized as being a light-inducible protein, and RT-PCR analysis showed that AL-P mRNA transcription only occurred in the leaves of the alfalfa plant and the alfalfa seedlings growth in lighted conditions. PCR was also used to amplify the DNA fragment encoding the AL-P promoter (AL-Pp) from genomic alfalfa DNA. PlantCARE analysis of the promoter sequence indicated that both a typical TATA box and a CAAT box were located in the promoter sequence, and some of the cis-elements that are responsible for light responsiveness were also identified within this promoter region. The AL-P gene promoter fused to the β-glucuronidase (GUS) reporter gene has been examined for expression in transgenic alfalfa seedlings. Our findings have a potential application in plant genetic engineering; the AL-Pp may be used to drive the expression of heterologous genes in transgenic alfalfa plants.