Microbial growth yield estimates from thermodynamics and its importance for degradation of pesticides and formation of biogenic non-extractable residues.

In biodegradation studies with isotope-labelled pesticides, fractions of non-extractable residues (NER) remain, but their nature and composition is rarely known, leading to uncertainty about their risk. Microbial growth leads to incorporation of carbon into the microbial mass, resulting in biogenic NER. Formation of microbial mass can be estimated from the microbial growth yield, but experimental data is rare. Instead, we suggest using prediction methods for the theoretical yield based on thermodynamics. Recently, we presented the Microbial Turnover to Biomass (MTB) method that needs a minimum of input data. We have estimated the growth yield of 40 organic chemicals (31 pesticides) using the MTB and two existing methods. The results were compared to experimental values, and the sensitivity of the methods was assessed. The MTB method performed best for pesticides. Having the theoretical yield and using the released CO2 as a measure for microbial activity, we predicted a range for the formation of biogenic NER. For the majority of the pesticides, a considerable fraction of the NER was estimated to be biogenic. This novel approach provides a theoretical foundation applicable to the evaluation and prediction of biogenic NER formation during pesticide degradation experiments, and may also be employed for the interpretation of NER data from regulatory studies.

Control of the direction of lamellipodia extension through changes in the balance between Rac and Rho activities.

The direction in which cells extend new motile processes, such as lamellipodia and filopodia, can be controlled by altering the geometry of extracellular matrix adhesive islands on which individual cells are cultured, thereby altering mechanical interactions between cells and the adhesive substrate [Parker (2002)]. Here we specifically investigate the intracellular molecular signals that mediate the mechanism by which cells selectively extend these processes from the corners of polygonal-shaped adhesive islands. Constitutive activation of the small GTPase Rac within cells cultured on square-shaped islands of fibronectin resulted in the elimination of preferential extension from corners. This loss of directionality was accompanied by a re-distribution of focal adhesions: the large focal adhesions normally found within the corner regions of square cells were lost and replaced by many smaller focal contacts that were distributed along the entire cell perimeter. Inhibition of the small GTPase, Rho, using C3 exoenzyme blocked lamellipodia extension entirely. However, inhibition of Rho signaling in combination with ectopic Rac activation rescued the corner localization of motile processes and focal adhesions. These results suggest that the ability of cells to sense their physical surroundings and respond by moving in a spatially oriented manner is mediated by a balance between Rho and Rac activities.

The S. cerevisiae Mag1 3-methyladenine DNA glycosylase modulates susceptibility to homologous recombination.

DNA glycosylases, such as the Mag1 3-methyladenine (3MeA) DNA glycosylase, initiate the base excision repair (BER) pathway by removing damaged bases to create abasic apurinic/apyrimidinic (AP) sites that are subsequently repaired by downstream BER enzymes. Although unrepaired base damage may be mutagenic or recombinogenic, BER intermediates (e.g. AP sites and strand breaks) may also be problematic. To investigate the molecular basis for methylation-induced homologous recombination events in Saccharomyces cerevisiae, spontaneous and methylation-induced recombination were studied in strains with varied MAG1 expression levels. We show that cells lacking Mag1 have increased susceptibility to methylation-induced recombination, and that disruption of nucleotide excision repair (NER; rad4) in mag1 cells increases cellular susceptibility to these events. Furthermore, expression of Escherichia coli Tag 3MeA DNA glycosylase suppresses recombination events, providing strong evidence that unrepaired 3MeA lesions induce recombination. Disruption of REV3 (required for polymerase zeta (Pol zeta)) in mag1 rad4 cells causes increased susceptibility to methylation-induced toxicity and recombination, suggesting that Pol zeta can replicate past 3MeAs. However, at subtoxic levels of methylation damage, disruption of REV3 suppresses methylation-induced recombination, indicating that the effects of Pol zeta on recombination are highly dose-dependent. We also show that overproduction of Mag1 can increase the levels of spontaneous recombination, presumably due to increased levels of BER intermediates. However, additional APN1 endonuclease expression or disruption of REV3 does not affect MAG1-induced recombination, suggesting that downstream BER intermediates (e.g. single strand breaks) are responsible for MAG1-induced recombination, rather than uncleaved AP sites. Thus, too little Mag1 sensitizes cells to methylation-induced recombination, while too much Mag1 can put cells at risk of recombination induced by single strand breaks formed during BER.

Nonspecific ulcers of the colon: a case presentation of ulceration of the transverse colon.

This paper describes a patient with nonspecific discrete ulcer of the transverse colon. The lesion was responsible for diarrhea which was cured by conservative resection. These uncommon ulcers, which mimic carcinoma radiographically and were previously thought to be uniformly fatal, may occasionally heal spontaneously. The patient has taken several drugs, some of which may have played a role in the pathogenesis of the ulcer, but the etiology of nonspecific colon ulcer remains uncertain.