SUMMARY: Ammonia assimilation in biovar strain RCR1001 (hereafter called ) appears to take place only through the glutamine synthetase/glutamate synthase pathway since () no glutamate dehydrogenase was detected in crude extracts of bacteria grown in different nitrogen sources, and () the growth rate on glutamine as a nitrogen source was faster than that observed on NHCI. In contrast to reports for other species, can definitely utilize NHC1 for growth. contains two glutamine synthetase isoenzymes, GSI and GSII, which can be detected in the presence of each other by differential heat stability, or separated by affinity chromatography or immunoabsorption with an antiserum raised against pure GSI. GSII does not cross-react with an anti-GSI antiserum. GSI was shown to be reversibly adenylylated and it was also shown that adenylylation inhibits the biosynthetic activity of this enzyme, in a similar way to that reported for glutamine synthetase and in contrast to that observed for glutamine synthetase of sp. strain ANU289. The apparent adenylylation level in different growth conditions changes from 21% to 99%, indicating a physiological role of this post-translational modification in the regulation of GSI activity. The intracellular concentration of GSI varies very little when is grown on different nitrogen sources (twofold when measured by the transferase assay, or fourfold when measured by ELISA). In addition, the concentration of mRNA specific for GSI in different nitrogen sources does not show appreciable differences. The intracellular concentration of GSII varies from a specific activity value higher than 1000 when is grown on glutamate or nitrate, to an undetectabie level when grown on NHC1. When NHC1 is added to a culture growing in glutamate, GSII activity is rapidly diluted out, suggesting a post-translational mechanism of enzyme inhibition or inactivation. Chloramphenicol prevents the disappearance of GSII activity, thus suggesting that protein synthesis is required for this process.


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