Characterization of a secondary uptake system for l-glutamate in Corynebacterium glutamicum

Abstract A new transport system for the uptake of l-glutamate was characterized in Corynebacterium glutamicum strain Δ glu, in which the previously described binding protein-dependent glutamate uptake system is not present. Kinetic characterization revealed a highly specific secondary transport system, dependent on sodium ions. Glutamate uptake showed Michaelis-Menten kinetics, with a K _m of 0.6 mM and a V _max of 15 nmol min⁻¹ (mg dw)⁻¹. For the co-transported sodium ions, a relatively low K _m of 3.3 mM was determined.     

Random Texts Do Not Exhibit the Real Zipf's Law-Like Rank Distribution

Background

Zipf''s law states that the relationship between the frequency of a word in a text and its rank (the most frequent word has rank , the 2nd most frequent word has rank ,…) is approximately linear when plotted on a double logarithmic scale. It has been argued that the law is not a relevant or useful property of language because simple random texts - constructed by concatenating random characters including blanks behaving as word delimiters - exhibit a Zipf''s law-like word rank distribution.

Methodology/Principal Findings

In this article, we examine the flaws of such putative good fits of random texts. We demonstrate - by means of three different statistical tests - that ranks derived from random texts and ranks derived from real texts are statistically inconsistent with the parameters employed to argue for such a good fit, even when the parameters are inferred from the target real text. Our findings are valid for both the simplest random texts composed of equally likely characters as well as more elaborate and realistic versions where character probabilities are borrowed from a real text.

Conclusions/Significance

The good fit of random texts to real Zipf''s law-like rank distributions has not yet been established. Therefore, we suggest that Zipf''s law might in fact be a fundamental law in natural languages.     

Corynebacterial protein kinase G controls 2-oxoglutarate dehydrogenase activity via the phosphorylation status of the OdhI protein

A novel regulatory mechanism for control of the ubiquitous 2-oxoglutarate dehydrogenase complex (ODH), a key enzyme of the tricarboxylic acid cycle, was discovered in the actinomycete Corynebacterium glutamicum, a close relative of important human pathogens like Corynebacterium diphtheriae and Mycobacterium tuberculosis. Based on the finding that a C. glutamicum mutant lacking serine/threonine protein kinase G (PknG) was impaired in glutamine utilization, proteome comparisons led to the identification of OdhI as a putative substrate of PknG. OdhI is a 15-kDa protein with a forkhead-associated domain and a homolog of mycobacterial GarA. By using purified proteins, PknG was shown to phosphorylate OdhI at threonine 14. The glutamine utilization defect of the delta pknG mutant could be abolished by the additional deletion of odhI, whereas transformation of a delta odhI mutant with a plasmid encoding OdhI-T14A caused a defect in glutamine utilization. Affinity purification of OdhI-T14A led to the specific copurification of OdhA, the E1 subunit of ODH. Because ODH is essential for glutamine utilization, we assumed that unphosphorylated OdhI inhibits ODH activity. In fact, OdhI was shown to strongly inhibit ODH activity with a Ki value of 2.4 nM. The regulatory mechanism described offers a molecular clue for the reduced ODH activity that is essential for the industrial production of 1.5 million tons/year of glutamate with C. glutamicum. Moreover, because this signaling cascade is likely to operate also in mycobacteria, our results suggest that the attenuated pathogenicity of mycobacteria lacking PknG might be caused by a disturbed tricarboxylic acid cycle.     

Calcium accumulation and regulation in Daphnia magna: Links with feeding, growth and reproduction

Calcium is involved in a wide variety of biological processes and has an important structural role in crustaceans. The present study aimed at exploring the possible link between Ca body concentrations and the ingestion rate and the role of soft tissue vs. total tissue Ca accumulation in Daphnia magna. D. magna was cultured for 21 days at different water Ca concentrations ranging from 3.4 to 32.5 mg/L. Every week Ca body concentrations (soft and total tissues), ingestion rate, growth, survival and reproduction were measured. Daily, algal food that was not deficient in Ca was supplied. Ca in the soft tissues represented 8 to 26% of the total Ca body concentrations. The ratio Ca in soft tissue/Ca in total tissue was generally not influenced by the Ca exposure concentration but decreased with time, i.e., age (from an average of 0.24 at day 7 to 0.09 at day 21). During week 1, a 54% decrease in Ca body concentrations was observed in daphnids exposed in medium with 3.4 mg/L Ca compared to those exposed to 32.5 mg/L. The concurrent decrease in ingestion rate was 14%. No significant differences among Ca treatments were observed during week 2 for ingestion rate and week 3 for calcium body concentrations. Also, no effects on growth and reproduction were observed, although these were expected at the lowest Ca concentration tested. It is hypothesised that Ca absorption from food in combination with an increased ingestion rate is used to maintain Ca homeostasis under Ca limiting conditions.