Global patterns of transcriptional activity of the symbiotic plasmid (pSym) of were studied under a variety of environmental conditions, including some relevant to the symbiotic process. P-labelled single-stranded complementary DNAs synthesized from total RNA were used as hybridization probes against an ordered collection of cosmid clones that covered the whole pSym. Our results showed that, under aerobic conditions, discrete regions of the pSym are differentially transcribed depending on the carbon and nitrogen sources employed. In general, poor carbon or nitrogen sources allowed greater expression than rich ones. Time-course experiments with the gene inducer genistein led us to the identification of new regions responsive to this flavonoid. Widespread transcription was observed during microaerobiosis, but not under aerobic conditions, indicating that oxygen concentration is a major effector of transcriptional activity in the pSym. This response was reduced, but not suppressed, in a mutant, indicating the location of regions whose transcription may depend on other oxygen-sensitive regulators. During symbiosis, almost the entire pSym was actively transcribed and the transcription pattern was similar to that observed during microaerobiosis. The experimental approach described allowed the identification and localization of specific regions in the pSym whose expression depends on defined environmental stimuli.


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