The cold case of state transition 7 (stt7) mutants of Chlamydomonas reinhardtii, solved by whole-genome sequencing.
The process of State Transitions corresponds to an STT7 kinase-driven redistribution of the transmembrane LHCII antenna proteins between PSII and PSI, which results from changes in their phosphorylation state. For the past two decades, two LHCII-kinase mutants, stt7-1 and stt7-9, have been instrumental in the study of state transitions in Chlamydomonas reinhardtii, the former being a null mutant for the kinase but quasi-sterile in crosses, while the latter, although fertile, has a leaky phenotype.  Here we further characterized their genetic lesion by using long-read sequencing, allowing us to recover the whole genome which we de novo assembled into their respective chromosomes. With these recent techniques we resolved unanticipated - but extensive - chromosomal rearrangements in the stt7-1 null mutant. Strikingly, we unveil a high level of aneuploidy in this original stt7-1 null mutant, which correlates with an increase in its cell size when compared with haploid cells used as controls, and with meiosis dysfunction. We then generated two new stt7 null mutants, one genetically derived from the original stt7­1, the other obtained by CRISPR-Cas9 technology. We thus produced more reliable null mutants for the LHCII-kinase, amenable to genetic crosses for the study of state transitions in a variety of genetic backgrounds.
The process of State Transitions corresponds to an STT7 kinase-driven redistribution of the transmembrane LHCII antenna proteins between PSII and PSI, which results from changes in their phosphorylation state. For the past two decades, two LHCII-kinase mutants, stt7-1 and stt7-9, have been instrumental in the study of state transitions in Chlamydomonas reinhardtii, the former being a null mutant for the kinase but quasi-sterile in crosses, while the latter, although fertile, has a leaky phenotype.  Here we further characterized their genetic lesion by using long-read sequencing, allowing us to recover the whole genome which we de novo assembled into their respective chromosomes. With these recent techniques we resolved unanticipated - but extensive - chromosomal rearrangements in the stt7-1 null mutant. Strikingly, we unveil a high level of aneuploidy in this original stt7-1 null mutant, which correlates with an increase in its cell size when compared with haploid cells used as controls, and with meiosis dysfunction. We then generated two new stt7 null mutants, one genetically derived from the original stt7­1, the other obtained by CRISPR-Cas9 technology. We thus produced more reliable null mutants for the LHCII-kinase, amenable to genetic crosses for the study of state transitions in a variety of genetic backgrounds.
Institut de Biologie Physico-Chimique