LAUSR

Chloroplast RNA processing requires a large number of nuclear-encoded RNA binding proteins (RBPs) that are imported post-translationally into the organelle. The chloroplast ribonucleoprotein 31A (CP31A) supports RNA editing at 13 sites and also supports the accumulation of multiple chloroplast mRNAs. The ndhF mRNA (coding for a subunit of the NDH complex) is most strongly affected in cp31a mutants. CP31A becomes particularly important at low temperatures, where it is essential for chloroplast development in young tissue. Next to two RNA recognition motifs (RRMs), CP31A has an N-terminal acidic domain (AD) that is phosphorylated at several sites. We investigated the function of the AD for CP31A's role in RNA metabolism and cold resistance. We demonstrate by point mutagenesis that the known phosphorylation sites within the acidic domain are irrelevant for any of the known functions of CP31A both at normal and at low temperatures. Even when the entire AD is removed, no effects on RNA editing were observed. By contrast, loss of the AD reduces CP31A's ability to stabilize the ndhF mRNA, which is paralleled by reduced NDH complex activity. Most importantly, AD-less CP31A lines display bleached young tissue in the cold. Together, these data show that the different functions of CP31A can be assigned to different regions of the proteins: the RRMs are sufficient to maintain RNA editing and to allow the accumulation of basal levels of ndhF, while chloroplast development in the cold critically depends on CP31A's AD.