> Research > Main Projects > MatK

The chloroplast splicing factor MatK

After endosymbiosis, the genome of the cyanobacterical endosymbiont has shrunken considerably. In present day angiosperms, only about 120 genes remain, most of which encode proteins with a function in photosynthesis. In addition, a considerable number of ribosomal proteins and RNA polymerase subunits can be found. There are currently only two conserved genes in angiosperm plastid chromosomes, for which no function is ascertained, one of them called matK. matK is unique in being the only gene for a non-ribosomal plastid-encoded RNA binding protein in higher plants and the only plastid reading frame with a putative role in RNA processing. MatK is related to intron maturases and possibly is involved in splicing of chloroplast group II introns. A chloroplast encoded splicing factor has long been proposed, because mutants like barley albostrians that are incapable of plastid translation fail to splice a specific subset of chloroplast introns, the so-called group IIA introns. However, no in vivo interaction of MatK with any intron was shown so far. Neither were MatK-interacting factors identified. And finally, close to nothing is known about matK’s expression and regulation.

This project asks: is MatK indeed the long sought chloroplast encoded splicing factor? What introns or other RNAs does it bind? How is matK regulated at the expression level?

To answer these questions, we have generated transplastomic tobacco lines by particle gun transformation that express N-and C-terminally tagged MatK proteins. The tags do not interfere with MatK function as homoplastomic mutants obtained are indistinguishable from wild-type plants. Using immunodetection of the tag, we could demonstrate that MatK is a stromal protein expressed preferentially in young seedlings. Bound RNAs were co-immunoprecipitated with tagged MatK and subsequently analyzed by RIP-Chip and dot-blot. This identified exclusively group IIA, but not other intron types or other mRNAs attached to MatK (Zoschke et al., 2010). Currently, we are generating matK hypomorphic mutants to directly test, whether the identified interactions are required for splicing. Furthermore, we have measured binding of MatK to its target introns across tobacco development in a quantitative way. We teamed up with the group of Ilka Axman at the ITB to model the MatK network mathematically. Insights from the model will be used to pinpoint regulatory roles of MatK in chloroplast gene expression.