Abstract
The first step of sulfur assimilation is the conversion of sulfate to adenosine 5'-phosphosulfate by the action of ATP sulfurylase. It has been reported that ATP sulfurylase activities are detected in the chloroplast and in the cytosol of Arabidopsis leaves. The Arabidopsis genome has four genes encoding ATP sulfurylase, ATPS1, ATPS2, ATPS3 and ATPS4. All of them are predicted to encode the protein with chloroplast transit peptides (TP) at their N-termini. Intracellular targeting of ATP sulfurylase isozymes was analyzed in the transgenic plants expressing ATPS-GFP fusion genes. The results suggested that ATPS2 localized both in the chloroplast and in the cytosol, while ATPS1, ATPS3 and ATPS4 localized exclusively in the chloroplast. Potential transcription start sites in the ATPS2 were identified by RLM-RACE. Most ATPS2 transcripts included the complete predicted TP sequence, thus would be translated into the chloroplast proteins. Several shorter transcripts lacking a complete TP sequence would be translated from in-frame ATG codon located in the TP sequence to generate the cytosolic proteins. Dual-targeting of ATPS2 might be achieved by multiple transcription initiation.
