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Genetic analysis reveals a key role for actin in tip growth in Neurospora crassa Virag, Aleksandra

Abstract

The body of a fdamentous fungus, the mycelium, is comprised of elongated cells called hyphae. During hyphal morphogenesis the elongated shape of a hypha is formed by polarized growth. Once initiated, polarized growth is restricted to hyphal tips and branching sites. Although some aspects of polarized growth, such as structural participants, are known, many others, such as control mechanisms, remain unclear. The goal of this study is to establish the position that actin maintains in the hierarchy of events leading to polarized growth in Neurospora crassa. The position of the mutation in the previously isolated mutant ccr-100 was identified by functional complementation, and the actin gene of the mutant was cloned and sequenced. The mutation occurred in the actin gene, and the ccr-100 strain was renamed act1. The act1 mutant had a different actin distribution, Spitzenkorper size and behaviour, and nuclear distribution. Additional actin mutants were obtained by taking advantage of the RIP (Repeat Induced Point mutations) mechanism. The actin genes of the actin RIP mutants were cloned and sequenced, and the mutants were analyzed in the same way as the act1 strain. The actin RIP mutants differed from the wild type in their colony and hyphal morphologies, and actin distributions. Interactions between act1 and seven other genes involved in polarized growth were demonstrated by the presence of epistasis. Based on the obtained results and existing models of tip growth, the Calcium-Actin-Phosphoinositide (CAP) model of tip growth is proposed. Actin has a central role in this model, and is anticipated to control the tip-high Ca2+ concentration gradient.

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