Fatty acid desaturases are nonheme, iron-containing, oxygen-dependent enzymes involved in regioselective introduction of double bonds in fatty acyl aliphatic chains (12). They are present in all groups of organisms, i.e., bacteria, fungi, plants, and animals, and they play a key role in the maintenance of the proper structure and functioning of biological membranes. Two general classes of fatty acid desaturases have been identified. (i) The acyl carrier protein (ACP) desaturases, which uses acyl-ACPs as substrates, are soluble plant enzymes and plastid-localized (15); (ii) the membrane-bound desaturases are found in a wide range of taxa and use acyl-CoA or acyl lipid substrates (13-15). They show different consensus motifs (15). The soluble class contains a consensus motif consisting of carboxylate and histidine ligands that coordinate an active site di-iron cluster, as revealed by the X-ray structure of the castor ⌬9 stearoyl-ACP desaturase (11). The integral membrane class contains a different consensus motif for the putative active site composed of three histidine-rich regions, which are presumably involved in iron binding (14). Unfortunately, information about substrate specificities, regioselectivities, and structure of membrane desaturases is scarce due to the technical limitations in obtaining large quantities of active forms of such enzymes.In a previous work, it was reported that the Bacillus subtilis des gene codes for a unique desaturase, Des (1). Transcription of the des gene is tightly controlled by a two-component regulatory system composed of a membrane-associated kinase, DesK, and a soluble transcriptional regulator, DesR (3). Activation of this pathway takes place when cells are shifted to low growth temperatures (2, 6). The B. subtilis desaturase is a polytopic membrane protein with six transmembrane domains and one membrane-associated domain, which likely represents a substrate-binding motif (7). This desaturase, which contains a tripartite motif of His essential for catalysis, is an acyl lipid desaturase, since it is able to introduce a double bond into the acyl chain of membrane glycerolipids (1, 6). It was previously shown that expression of the des gene in Escherichia coli resulted in desaturation of the palmitic moieties of the membrane phospholipids to give cis-5-hexadecenoic acid, indicating that the B. subtilis desaturase is a ⌬5 acyl lipid desaturase (1). However, Weber et al. (17) recently reported that four unsaturated fatty acid (UFA) species differing in length, branching pattern, and position of the double bond were synthesized in B. subtilis JH642. Under the same conditions, none of these fatty acids species was detected in a B. subtilis isogenic strain possessing a des null mutation, showing that their production is strictly dependent on the B. subtilis desaturase (10). On the basis of these data, Weber et al. (17) questioned whether the B. subtilis desaturase was a ⌬5 desaturase and concluded that this enzyme was able to introduce double bonds into different positions (⌬5, ⌬7,...
Thirty-four isolates ofAspergillus flavus obtained from the main Argentinian corn production area were tested for their ability to produce both cyclopiazonic acid (CPA) on corn and on liquid media and aflatoxins on corn.Aflatoxins and CPA were quantified by comparison with standards. The last one was confirmed by mass spectrometry.All but one of the isolates produced CPA on liquid medium in a range between 3120 to 62500 μg/kg, 27/34 isolates produced CPA on corn at levels ranging from 833 to 10000 μg/kg and 5/34 isolates produced aflatoxin B1 in a range between 29 to 115 μg/kg. According to these findings, the percentage ofAspergillus flavus isolates with CPA production ability and their levels of CPA production were higher than the observed elsewhere.It was observed significant differences (p<0,01) between CPA production on corn (median: 1761 μg/Kg) and in liquid medium (median: 27950 μg/Kg). These data represent the first report of the co-production of CPA and aflatoxin B1 by isolates ofAspergillus flavus obtained from corn in Argentina.
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