plant surface microbiology.pdf

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Marjatta Raudaskoski, Mika Tarkka and Sara Niini
the activation of the dynein motor molecule and in the binding of dynein to
membranes (King and Schroer 2000).
Dynein heavy chain encoding genes have been characterized from A. nidulans
with the help of a temperature-sensitive mutation nudA that affects
nuclear distribution, and from N. crassa as a morphological mutation with
curly hyphae named ro-1. In the temperature-sensitive mutation of the nudA
gene the nuclear movement from conidia into the germ tube failed and small
hyphal colonies were formed at restrictive temperature (Xiang et al. 1994). In
the ro-1 mutant of N. crassa, nuclear aggregates formed in the hyphae at some
distance from the hyphal tip (Plamann et al. 1994). The cloning and sequencing
of the nudA and ro-1 genes revealed that they both encode the cytoplasmic
dynein heavy chains with ATP-binding motor domain at the C-terminus.
Later, the dynein heavy chains have been identified from N. haematococca
(Inoue et al. 1998), and U. maydis (Straube et al. 2001), by using degenerate
oligonucleotide primers. Interestingly, this approach has led to the identification
of a dynein with split motor domain in U. maydis, in which the N-terminus
of the motor domain including the four ATP binding regions is encoded
by dyn1 and the MT-binding part by dyn2 gene. Some of the ro-1-like phenotypic
N. crassa mutants carry mutations in genes encoding subunits of the
dynactin complex (Bruno et al. 1996). This has facilitated the isolation of ro-4
and ro-3 genes encoding the actin-related protein,Arp1 (Plamann et al. 1994),
and p150 Glued (Tinsley et al. 1996). Several other ro genes, such as ro-2, ro-7 and
ro-12, encoding different subunits of the dynactin complex have been identified
(Lee et al. 2001).
By comparing the effects of the deletion of kinesin (Nkin), or dynein (ro-1),
and both proteins on nuclear distribution, vesicle transport, secretion and
vacuole formation in N. crassa hyphae, it was concluded that conventional
kinesin indeed is responsible for the apical transport of vesicles destined for
secretion, whereas dynein is responsible for nuclear movements and transport
of vacuole precursors in the opposite direction. The latter phenomenon
is suggested to support the formation of vacuoles in the basal part of N. crassa
hyphae (Seiler et al. 1999), while conventional kinesin encoded by kin2 was
shown to be responsible for the accumulation of vacuoles to the basal part of
in U. maydis dikaryotic hyphae (Steinberg et al. 1998). These results indicate
that mutations in different motor molecules may result in the same phenotype
in fungi belonging to different taxa, the phenotype in this case being the
vacuolation of the basal part of a hypha. Interestingly, in a dynein-deficient
mutant of N. haematococca, astral-like arrays of cytoplasmic MTs radiating
from nuclear spindle pole bodies were missing, which probably causes the
clustered nuclear distribution in the mutant hyphae. In filamentous fungi, the
astral MTs are suggested to be responsible for post-mitotic nuclear migration
and anchoring of the interphase nuclei to membrane structures (Aist and
Bayles 1988; Salo et al. 1989; Raudaskoski et al. 1991; Morris et al. 1995; Inoue
et al. 1998; Raudaskoski 1998).