Transcriptional Control in Marine Copiotrophic and Oligotrophic Bacteria with Streamlined Genomes

Bacteria often respond to environmental stimuli using transcriptional control, but this may not be the case for marine bacteria such as "Candidatus Pelagibacter ubique," a cultivated representative of the SAR11 clade, the most abundant organism in the ocean. This bacterium has a small, streamlined genome and an unusually low number of transcriptional regulators, suggesting that transcriptional control is low in Pelagibacter and limits its response to environmental conditions. Transcriptome sequencing during batch culture growth revealed that only 0.1% of protein-encoding genes appear to be under transcriptional control in Pelagibacter and in another oligotroph (SAR92) whereas >10% of genes were under transcriptional control in the copiotrophs Polaribacter sp. strain MED152 and Ruegeria pomeroyi When growth levels changed, transcript levels remained steady in Pelagibacter and SAR92 but shifted in MED152 and R. pomeroyi Transcript abundances per cell, determined using an internal RNA sequencing standard, were low (<1 transcript per cell) for all but a few of the most highly transcribed genes in all four taxa, and there was no correlation between transcript abundances per cell and shifts in the levels of transcription. These results suggest that low transcriptional control contributes to the success of Pelagibacter and possibly other oligotrophic microbes that dominate microbial communities in the oceans.