Amino acid and nucleotide pools were measured in nitrogenase-containing Rhodospirillum rubrum cultures during NH4+-or dark-induced inactivation (switch-off) of the Fe protein. A big increase in the glutamine pool size preceded NH4+ switch-off of nitrogenase activity, but the glutamine pool rqmained unchanged during dark switch-off. Furthermore, methionine sulfoximine had no effect on the rate of dark switch-off, suggesting that glutamine plays no role in this process. In the absence of NH4' azaserine, an inhibitor of glutamate synthate, raised glutamine pool levels sufficiently to initiate switch-off in vivo. While added NH4L substantially increased the size of the nucleotide pools in N-limited cells, the kinetics of nucleotide synthesis were all similar and followed (rather than preceded) Fe protein inactivatioi. Darkness had little effect on nucleotide pool sizes. Glutamate pool sizes were also found to be important in NH4' switch-off because of the role of this molecule as a glutamine precursor. Much of the diversity reported in the observations on NH4' switch-off appears to be due to variations in glutamate pool sizes prior to the NH4+ shock. The nitrogen nutritional background is an important factor in determnining whether darkness initiates nitrogenase switch-off; however, no link has yet been established between this and NH4' (glutamine) switch-off.The nonsulfur photosynthetic bacteria are among a group of N2-fixing bacteria with nitrogenase activity that is subject to regulation by NH4'. Carithers et al. (4) and Nordlund and Eriksspn (17) showed that the inhibitory effect of NH4' on nitrogenase activity (switch-off) is related to the covalent modification and concurrent inactivation of the nitrogenase Fe protein extracted from glutamate-and N2-grown cells (13,14,18). Similar to the inactive Fe protein isolated from glutamate-grown cells, Fe protein extracted from NH4+-inhibited cells (4, 17) was inactive and could be reactivated by a membrane-bound activating enzyme (13, 18). Switch-off of nitrogenase activity by NH4' catalyzes the covalent modification of one of the two Fe protein subunits (6, 23) with one molecule of ADP-ribose (22).The role of NH4' as an in vivo signal to activate an Fe protein-inactivating enzyme has been the subject of numerous investigations. Glutamine mimics NH4' (16) and methionine sulfoximine (MSX), an inhibitor of glutamine synthetase (GS), prevents NH4' switch-off in Rhodospirillum rubrum (5,25,30) and all other N2 fixers that are capable of switch-off regulation (3, 9). 6-Diazo-5-oxo-L-norleucine (30) and azaserine (7) are inhibitors of glutamate synthase (GOGAT), and both initiate switch-off in the absence of NH4', presumably by blocking the normal flux of nitrogen through the GS-GOGAT pathway. This results in a glutamine buildup sufficient to initiate switch-off.The confirmed and extended to show that R. rubrum could rapidly reverse this process in the light (10). It is not readily apparent what glutamine and darkness have in common that leads to the activation of the Fe...

Aims: To find new insecticidal antibiotics from marine micro-organisms. Methods and Results: Strains isolated from seawater and sea sediments from Beidiahe and Dagang of the east coast of China were screened for their insecticidal qualities. The screening was carried out using bioassay of brine shrimp and the insect pest Helicoverpa armigera. The fermentation, preliminary extraction and isolation of Streptomyces sp.173 were carried out. Conclusions: In total 331 isolates were examined through bioassay of brine shrimp and 40 isolates (12AE08%) showed potential insecticidal activities. Of the 40 isolates, one isolate, designated Streptomyces sp.173, was found to have strong insecticidal activity against both brine shrimp and H. armigera, similar to that of avermectin B 1 . Significance and Impact of the Study: The isolated Streptomyces sp.173 has great insecticidal potency. This work indicated that marine micro-organisms could be an important source of insecticidal antibiotics and the improved anti-brine shrimp bioassay is suitable for primary screening.

Aims:  To differentiate pathogenic and nonpathogenic Edwardsiella tarda strains based on the detection of type III secretion system (T3SS) gene using polymerase chain reaction (PCR). Methods and Results:  Primers were designed to amplify Edw. tarda T3SS component gene esaV, catalase gene katB, haemolysin gene hlyA and 16S rRNA gene as an internal positive control. Genomic DNAs were extracted using a commercial isolation kit from 36 Edw. tarda strains consisting of 18 pathogenic and 18 nonpathogenic strains, and 50 ng of each DNA was used as the template for PCR amplification. PCR was performed with a thermocycler (TaKaRa TP600) in a 25‐μl volume. Products of esaV were detected in all pathogenic strains, but not in nonpathogenic strains; katB was detected in all pathogenic strains and one of nonpathogenic strains; hlyA was not detected in any strains. Conclusions:  The detection of esaV gene can be used for the assessment of pathogenic Edw. tarda strains. Significance and Impact of the Study:  The strategy using T3SS gene as the virulence indicator provides a useful tool for the clinical assessment of pathogenic Edw. tarda strains and prediction of edwardsiellosis risk in fish culture environments.