The Arabidopsis pentatricopeptide repeat protein LPE1 and its maize ortholog are required for translation of the chloroplast psbJ RNA. 
The expression of chloroplast genes relies on a host of nucleus-encoded proteins.  Identification of such proteins and elucidation of their functions are ongoing challenges.  We used ribosome profiling to revisit the function of the pentatricopeptide repeat protein LPE1, reported to stimulate translation of the chloroplast psbA mRNA in Arabidopsis.  Mutation of the maize LPE1 ortholog causes a photosystem II (PSII) deficiency and a defect in translation of the chloroplast psbJ open reading frame (ORF) but has no effect on psbA expression.  To reflect this function, we named the maize LPE1 ortholog Translation of psbJ 1 (TPJ1).  Arabidopsis lpe1 mutants likewise exhibit a loss of psbJ translation, and have, in addition, a decrease in psbN translation.  We detected a small decrease in ribosome occupancy on the psbA mRNA in Arabidopsis lpe1 mutants, but ribosome profiling analyses of other PSII mutants (hcf107 and hcf173) in conjunction with in vitro RNA binding data strongly suggest that this is a secondary effect of their PSII deficiency.  We conclude that maize TPJ1 promotes PSII synthesis by activating translation of the psbJ ORF, that this function is conserved in Arabidopsis LPE1, and that an additional role for LPE1 in psbN translation contributes to the PSII deficiency in lpe1 mutants.