Interaction of PALE CRESS with PAP2/pTAC2 and PAP3/pTAC10 affects the accumulation of plastid-encoded RNA polymerase complexes in Arabidopsis.

The transcription of photosynthesis genes in chloroplasts is largely mediated by the plastid-encoded RNA polymerase (PEP), which resembles prokaryotic-type RNA polymerases, but with plant-specific accessory subunits known as plastid transcriptionally active chromosome proteins (pTACs) or PEP-associated proteins (PAPs). However, whether additional factors are involved in the biogenesis of PEP complexes remains unknown. Here, we investigated the function of an essential gene, PALE CRESS (PAC), in the accumulation of PEP complexes in chloroplasts. We established that an Arabidopsis leaf variegation mutant, variegated 6-1 (var6-1), is a hypomorphic allele of PAC. Unexpectedly, we revealed that a fraction of VAR6/PAC is associated with thylakoid membranes, where it interacts with PEP complexes. The accumulation of PEP complexes is defective in both var6-1 and the null allele var6-2. Further protein interaction assays confirmed that VAR6/PAC interacts directly with the PAP2/pTAC2 and PAP3/pTAC10 subunits of PEP complexes. Moreover, we generated viable hypomorphic alleles of the essential gene PAP2/pTAC2, and revealed a genetic interaction between PAC and PAP2/pTAC2 in photosynthesis gene expression and PEP complex accumulation. Our findings establish that VAR6/PAC affects PEP complex accumulation through interactions with PAP2/pTAC2 and PAP3/pTAC10, and provide new insights into the accumulation of PEP and chloroplast development.