Involved, directly or indirectly, in the processing of the chloroplast encoded psbD mRNA to its mature form, acting via the 5'-UTR of the psbD mRNA. The last 588 amino acids of the protein are sufficient to confer stability on the transcript in vivo. Part of 2 complexes of about 600 and less than 2000 kDa, both of which also contain non-polysomal RNA.