BioProject

Bacteria respond to stimuli in the environment using transcriptional control, but this may not be the case for most marine bacteria having small, streamlined genomes. Candidatus Pelagibacter ubique, a cultivated representative of the SAR11 clade, which is the most abundant clade in the oceans 4, has a small, streamlined genome and possesses an unusually small number of transcriptional regulators. This observation leads to the hypothesis that transcriptional control is low in Pelagibacter and limits its response to environmental conditions. However, the extent of transcriptional control in Pelagibacter is unknown. Here we show that transcriptional control is extremely low in Pelagibacter and another oligotroph (SAR92) compared to two marine copiotrophic bacterial taxa, Polaribacter MED152 and Ruegeria pomeroyi. We found that ~0.1% of protein-encoding genes in Pelagibacter are under transcriptional control compared to >10% of genes in other marine bacteria. Regardless of the growth condition, the same genes were highly expressed while most genes were always expressed at very low levels. Quantitative RNA sequencing revealed that abundances of most Pelagibacter transcripts were <0.01 copies per cell whereas transcript abundances were 1 to 10 copies per cell in some other bacteria. Our results demonstrate that Pelagibacter can change growth without shifts in transcript levels, suggesting that transcriptional control plays a minimal role in the adaptive strategy for one of the most successful organisms in the biosphere. Overall design: Bacteria were grown in batch culture and sampled twice during the initial, rapid phase of exponential growth and twice during the phase of slower growth that followed.