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dictyNews Volume 39 Number 04
dictyNews
Electronic Edition
Volume 39, number 4
February 8, 2013
Please submit abstracts of your papers as soon as they have been
accepted for publication by sending them to dicty@northwestern.edu
or by using the form at
http://dictybase.org/db/cgi-bin/dictyBase/abstract_submit.
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Abstracts
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A Rare Combination of Ribonucleotide Reductases in the Social
Amoeba Dictyostelium discoideum
Mikael Crona, Lotta Avesson, Margareta Sahlin, Daniel Lundin,
Andrea Hinas, Ralph Klose, Fredrik Soderbom, and Britt-Marie Sjoberg
Journal of Biological Chemistry, in press
Ribonucleotide reductases (RNRs) catalyze the only pathway for de novo
synthesis of deoxyribonucleotides needed for DNA replication and repair.
The vast majority of eukaryotes encodes only a class I RNR, but
interestingly some eukaryotes, including the social amoeba Dictyostelium
discoideum, encode both a class I and a class II RNR. The amino acid
sequence of the D. discoideum class I RNR is similar to other eukaryotic
RNRs, whereas that of its class II RNR is most similar to the monomeric
class II RNRs found in Lactobacillus spp. and a few other bacteria. Here
we report the first study of RNRs in a eukaryotic organism that encodes
class I and class II RNRs. Both classes of RNR genes were expressed in
D. discoideum cells, although the class I transcripts were more abundant
and strongly enriched during mid development compared to the class II
transcript. The quaternary structure, allosteric regulation, and properties of
the diiron-oxo/radical cofactor of D. discoideum class I RNR are similar to
the mammalian RNRs. Inhibition of D. discoideum class I RNR by
hydroxyurea resulted in a 90% reduction in spore formation and decreased
the germination viability of the surviving spores by 75%. Class II RNR could
not compensate for class I inhibition during development, and excess of
vitamin B12 coenzyme essential for class II activity did not improve spore
formation. We suggest that class I is the principal RNR during D. discoideum
development and growth, and is important for spore formation, possibly by
providing dNTPs for mitochondrial replication.
Submitted by Fredrik Soderbom [fredrik.soderbom@icm.uu.se]
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Robustness of self-organizing chemoattractant field arising from precise
pulse-induction of its breakdown enzyme: a single cell level analysis of
PDE expression in Dictyostelium
Noritaka Masakià, Koichi Fujimotoà, Mai Honda-Kitahara, Emi Hada and
Satoshi Sawai
à Both authors contributed equally to this work.
Graduate School of Arts and Sciences, The University of Tokyo
Biophysical Journal, in press
The oscillation of chemoattractant cyclic AMP (cAMP) in Dictyostelium
discoideum is a collective phenomenon that occurs when the basal level
of extracellular cAMP exceeds a threshold and invokes cooperative mutual
excitation of cAMP synthesis and secretion. In order for pulses to be relayed
from cell to cell repetitively, secreted cAMP must be cleared and brought
down to the sub-threshold level. One of the main determinants of the
oscillatory behavior is thus how much extracellular cAMP is degraded by
extracellular phosphodiesterase (PDE). To date, the exact nature of its
gene regulation remains elusive. Here, we performed live imaging analysis
of mRNA transcripts for pdsA - the gene encoding extracellular
phosphodiesterase. Our analysis revealed that pdsA is up-regulated during
the rising phase of the cAMP oscillations. Furthermore we show by analyzing
isolated cells that its expression is strictly dependent on the presence of
extracellular cAMP. It is induced only at ~1nM extracellular cAMP, which is
almost identical to the threshold concentration for the cAMP relay response.
The observed precise regulation of PDE expression together with
degradation of extracellular cAMP by PDE form a dual positive and negative
feedback circuit, and model analysis shows that this sets the cAMP level
near the threshold concentration for the cAMP relay response for a wide
range of adenylyl cyclase activity. The overlap of the thresholds could allow
oscillations of chemoattractant cAMP to self-organize at various starving
conditions making its development robust to fluctuating environment.
Submitted by Satoshi Sawai [cssawai@mail.ecc.u-tokyo.ac.jp]
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[End dictyNews, volume 39, number 4]
