No abstract

Manganese (Mn) is key to oxygenic photosynthesis as it catalyzes the splitting of water in photosystem II and functions as cofactor of multiple enzymes. A single ABC-type transporter, MntCAB, is so far established for the uptake of the metal under limiting conditions in cyanobacteria. It is unknown, how Mn is imported under replete conditions. We identified two proteins in the model cyanobacterium Synechocystis sp. PCC 6803, which are homologous to the unknown protein family 0016 (UPF0016) member manganese exporter (Mnx). In contrast to Mnx, which consists of six transmembrane domains, the new candidate proteins contain only three transmembrane domains. Hence, we named them hemi manganese exporter (Hmx) 1 and 2. Knock-out mutants in hmx1 and/or hmx2 showed sensitivity toward low Mn supplementation, and reduced intracellular Mn pools. Additional deletion of mntC hindered the cells to thrive unless external Mn was added and enhanced the depletion of their intracellular Mn pool. In accordance with the observed localization of Hmx1 and Hmx2 in the plasma membrane, we postulate a Mn uptake function for a heteromeric Hmx1/2 across the plasma membrane under a wide range of Mn concentrations and a supporting role for the MntCAB system under Mn-limiting conditions. On the basis of their phylogenies, we propose that Hmx1 and Hmx2 are the ancestral progenitors of eukaryote-type UPF0016 proteins with six transmembrane domains. The Mn transport function of Hmx1/2 underscores this as fundamental ancient feature of the UPF0016 family. Likely, Hmx1 and Hmx2 coevolved with the internalization of the oxygen-evolving complex.

A percepção de sons é hoje uma das variáveis de investigação nos estudos de aprendizagem do sistema fonológico de língua estrangeira. Nas ultimas décadas, diversas teorias ou modelos foram propostos na tentativa de explicar o quanto a experiência linguistica com a língua materna afeta tanto a percepção quanto a produção de fala não nativa. Este artigo apresenta um dos modelos de percepção de sons de língua estrangeira mais influentes nos estudos de fonética e fonologia, o Modelo de Assimilação Perceptual proposto por Best em 1995. Em 2007, Best e Tyler ampliaram o modelo abrangendo a aprendizagem perceptual. Além da apresentação dos dois modelos, o artigo também propõe sugestões de como utilizá-los empiricamente.