Title

Tyrosine phosphorylation of NIMXcdk1 and the regulation of septum formation in Aspergillus nidulans

Date of Completion

January 2001

Keywords

Biology, Genetics|Biology, Microbiology

Degree

Ph.D.

Abstract

In Aspergillus nidulans, germinating conidia undergo multiple rounds of nuclear division before forming a septum. Previous genetic results suggest that the ability to separate nuclear division and septum formation depends upon a threshold level of activity of the cyclin-dependent kinase NIMXcdk1. Mutations in nimX and nimT , the gene encoding the NIMXcdk1-activating phosphatase, have revealed that Tyr-15 phosphorylation is important for determining the timing of the formation of the first septum. Here, we describe two approaches designed to test the hypothesis that dephosphorylation of NIMXcdk1 at Tyr-15 determines the timing of septum formation in A. nidulans. First, a screen for suppressors of nimT23 (snt) was designed in order to identify additional components of the pathway regulating septum formation. We show that the snt mutants are defective in the temporal regulation of septum formation and cell cycle checkpoints. Molecular characterization of sntA shows that it is allelic to the previously described ankA gene, which encodes the NIMXcdk1 Tyr-15 kinase. Additional experiments described in this study show that nutritional conditions modulate the timing of septum formation and the snt phenotypes. A model is proposed which suggests that the timing of septum formation is influenced by DNA damage and glucose availability via the snt gene products. Second, a reverse genetic approach was used to identify and characterize a potential negative regulator of NIMT, the 14-3-3 protein ARTA. We tested the hypothesis that ARTA is a dose-dependent suppressor of septation using an over-expression strain. Increased ARTA levels resulted in defects in polarized growth and conidiation, but had no effect on the timing of septum formation. ^