SucA-dependent uptake of sucrose across the outer membrane of Caulobacter crescentus

Caulobacter crescentus is an aquatic Gram-negative bacterium that lives in nutrient-poor environments. Like several other aquatic and phytopathogenic bacteria, Caulobacter cells have a relatively large number of genes predicted to encode TonB-dependent receptors (TBDRs). TBDRs transport nutrients across the outer membrane using energy from the proton motive force. We identified one TBDR gene, sucA, which is situated within a cluster of genes predicted to encode a lacI-family transcription factor (sucR), amylosucrase (sucB), fructokinase (sucC), and an inner membrane transporter (sucD). Given its genomic neighborhood, we proposed that sucA encodes a transporter for sucrose. Using RT-qPCR, we determined that expression of sucABCD is strongly induced by sucrose in the media and repressed by the transcription factor, SucR. Furthermore, cells with a deletion of sucA have a reduced uptake of sucrose. Although cells with a non-polar deletion of sucA can grow with sucrose as the sole carbon source, cells with a polar deletion that eliminates expression of sucABCD cannot grow with sucrose as the sole carbon source. These results show that the suc locus is essential for sucrose utilization while SucA functions as one method of sucrose uptake in Caulobacter crescentus. This work sheds light on a new carbohydrate utilization locus in Caulobacter crescentus.