RNA-binding activity of the matK protein encoded by the chloroplast trnK intron from mustard (Sinapis alba L.).

The chloroplast trnK gene for tRNALys(UUU) from mustard contains a 2574 bp group II intron with a long open reading frame for 524 amino acids. The encoded polypeptide appears to be structurally related to mitochondrial maturases which are involved in splicing. To study the properties of the intron encoded protein, we overexpressed the trnK ORF as a beta-galactosidase fusion protein in E. coli and carried out RNA-protein binding experiments with crude bacterial extracts and the purified fusion protein. Both gel-shift and UV-crosslinking experiments revealed preferential binding to the trnK precursor transcript. Of two other RNA probes containing chloroplast group II introns, the trnG precursor was recognized by the trnK ORF protein, but the rps16 precursor was not. Competition binding experiments indicate that G-residues seem to play a role in RNA-protein interaction. RNA-binding activity of the trnK intron encoded polypeptide is consistent with its suggested function as a plastid maturase, hence justifying the assignment matK for this gene.