Biology subjects

Williams-Carrier, R.

Publications and source records attributed to Williams-Carrier, R..

3 recordsLinked to original sources

bioRxiv2024

Opposing action of photosystem II assembly factors RBD1 and HCF136 underlies light-regulated psbA translation in plant chloroplasts

The D1 reaction center protein of photosystem II (PSII) is subject to light-induced damage. In plants, D1 photodamage activates translation of the chloroplast psbA mRNA encoding D1, providing D1 for PSII repair. Genetic data have implicated three D1 assembly factors in the regulatory mechanism: HCF244 and RBD1 activate psbA translation whereas HCF136 represses psbA translation in the dark. To clarify the regulatory circuit, we analyzed psbA ribosome occupancy in dark-adapted and illuminated rbd1 and hcf136;rbd1 double mutants in Arabidopsis, and in Zm-hcf244 and Zm-hcf136;Zm-hcf244 double mutants in maize. The results show that RBD1 is required for the light-induced recruitment of ribosomes to psbA mRNA but has little effect on psbA ribosome occupancy in the dark. Furthermore, RBD1 is dispensable for psbA translation when HCF136 is absent, indicating that RBD1 activates psbA translation indirectly, by inhibiting HCF136s repressive effect. By contrast, HCF244 is required to recruit ribosomes to psbA mRNA in light, dark, and in the absence of HCF136. These results demonstrate that the step in D1 assembly that is mediated by RBD1 is central to the perception of the D1 photodamage that triggers D1 synthesis, that RBD1 activates psbA translation in the light by relieving the repressive effect of an HCF136-dependent assembly intermediate, and that HCF244 acts independently of these events to activate psbA translation. The results implicate a feature of nascent D1 that is affected by both HCF136 and RBD1 as the signal that reports D1 photodamage to regulate psbA translation rate as needed for PSII repair. Significance StatementThe D1 subunit of photosystem II (PSII) is damaged by light and must be replaced with nascent D1 to maintain photosynthesis. In plants, D1 photodamage triggers D1 synthesis by activating translation of chloroplast psbA mRNA encoding D1. Our results demonstrate that an antagonistic interplay between two photosystem II assembly factors, HCF136 and RBD1, is at the core of the signal transduction process that senses light-induced D1 damage to regulate psbA translation. These findings implicate a feature of nascent D1 that is affected by both HCF136 and RBD1 as the signal that reports D1 photodamage to tune psbA translation as needed for PSII repair. The results elucidate an assembly-coupled translational rheostat that maintains photosynthesis in the face of PSII photodamage.

plant biology↗

bioRxiv2024

Light activates psbA translation in plants by relieving inhibition of translation factor HCF173 by the psbA ORF in cis

The D1 reaction center protein of photosystem II is subject to photooxidative damage. Photodamaged D1 must be replaced with newly synthesized D1 to maintain photosynthesis. In plant chloroplasts, D1 synthesis is coupled to D1 photodamage via regulated translation initiation on psbA mRNA, which encodes D1. Mechanisms underlying this coupling are unclear. We show by analysis of reporter constructs in tobacco that the psbA translational activators HCF173 and HCF244 activate via cis-elements in the psbA UTRs and that the 5 UTR sequence bound by HCF173 is essential for psbA expression. However, the psbA UTRs are not sufficient to program light-regulated translation. Instead, the psbA open reading frame acts in cis to repress translation initiation, and D1 photodamage relieves this repression. A truncated HCF173 mutant conditions constitutively high psbA ribosome occupancy in the dark, implicating HCF173 as a mediator of the repressive signal. We propose a model that is informed by structures of the Complex I assembly factor CIA30/NDUFAF1 positing that D1 photodamage relieves a repressive cotranslational interaction between nascent D1 and HCF173s CIA30 domain, and that the D1 assembly factor HCF136 promotes this interaction. These findings elucidate a translational rheostat that maintains photosynthesis in the face of inevitable photosystem II photodamage.

plant biology↗

bioRxiv2018

The dynamic association of SPO11-1 with conformational changes of meiotic axial elements in maize

Meiotic double-strand breaks (DSBs) are generated by the evolutionarily conserved SPO11 complex in the context of chromatin loops that are organized along axial elements (AEs) of chromosomes. However, how DSBs are formed with respect to chromosome axes and the SPO11 complex remains unclear in plants. Here, we confirm that DSB and bivalent formation are defective in maize spo11-1 mutants. Super-resolution microscopy demonstrates dynamic localization of SPO11-1 during recombination initiation, with variable numbers of SPO11-1 foci being distributed in nuclei but similar numbers of SPO11-1 foci being found on AEs. Notably, cytological analysis of spo11-1 meiocytes revealed an aberrant AE structure. At leptotene, AEs of wild-type and spo11-1 meiocytes were similarly curly and discontinuous. However, during early zygotene, wild-type AEs become uniform and exhibit shortened axes, whereas the elongated and curly AEs persisted in spo11-1 mutants, suggesting that loss of SPO11-1 compromised AE structural maturation. Our results reveal an interesting relationship between SPO11-1 loading onto AEs and the conformational remodeling of AEs during recombination initiation. Author SummaryMeiosis is essential during sexual reproduction to produce haploid gametes. Recombination is the most crucial step during meiotic prophase I. It enables pairing of homologous chromosomes prior to their reductional division and generates new combinations of genetic alleles for transmission to the next generation. Meiotic recombination is initiated by generating DNA double-strand breaks (DSBs) via SPO11, a topoisomerase-related enzyme. The activity, timing and location of this DSB machinery must be controlled precisely, but how this is achieved remains obscure. Here, we show dynamic localization of SPO11-1 on chromatin during meiotic initiation in maize, yet a similar number of SPO11-1 is able to load onto axial elements (AEs), which accompanies a structural change of the AEs of wild-type meiotic chromosomes. Interestingly, loss of SPO11-1 not only affects DSB formation but also impairs structural alterations of AEs, resulting in abnormally long and curly AEs during early meiosis. Our study provides new insights into SPO11-1 localization during recombination initiation and suggests an intimate relationship between DSB formation and AE structural changes.

molecular biology↗