dictyNews
Electronic Edition
Volume 39, number 4
February 8, 2013

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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 Söderbom, and Britt-Marie Sjöberg


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 Söderbom [[log in to unmask]]
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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 Masaki‡, Koichi Fujimoto‡, Mai Honda-Kitahara, Emi Hada and 
Satoshi Sawai

‡ 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 [[log in to unmask]]
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[End dictyNews, volume 39, number 4]