Transcriptional Regulation Buffers Gene Dosage Effects on a Highly Expressed Operon in Salmonella

Highly expressed genes are commonly located close to the origin of replication of bacterial chromosomes (OriC). This location skew is thought to reflect selective advantages associated with gene dosage effects during the replication cycle. The expression of constitutively expressed genes can vary up to fivefold based on chromosomal location, but it is not clear what level of variation would occur in naturally regulated operons. We tested the magnitude of the chromosome location effect using EF-Tu (tufA, tufB), an abundant protein whose cellular level correlates with, and limits, the maximum growth rate. We translocated the Salmonella tufB operon to four locations across the chromosome. The distance from OriC had only a small effect on growth rate, consistent with this operon having the natural ability to upregulate expression and compensate for reduced gene dosage. In contrast, when the total EF-Tu concentration was limiting for the growth rate (tufA deleted), we observed a strong gene dosage effect when tufB was located further from OriC. However, only a short period of experimental evolution was required before the bacteria adapted to this EF-Tu starvation situation by acquiring genetic changes that increased expression levels from the translocated tufB gene, restoring growth rates. Our findings demonstrate that, at least for the tufB operon, gene dosage is probably not the dominant force selecting for a chromosomal location close to OriC. We suggest that the colocation of highly expressed genes close to OriC might instead be selected because it enhances their coregulation during various growth states, with gene dosage being a secondary benefit. IMPORTANCE A feature of bacterial chromosomes is that highly expressed essential genes are usually located close to the origin of replication. Because bacteria have overlapping cycles of replication, genes located close to the origin will often be present in multiple copies, and this is thought to be of selective benefit where high levels of expression support high growth rate. However, the magnitude of this selective effect and whether other forces could be at play are poorly understood. To study this, we translocated a highly expressed essential operon, tufB, to different locations and measured growth fitness. We found that transcriptional regulation buffered the effects of translocation and that even under conditions where growth rate was reduced, genetic changes that increased the expression of tufB were easily and rapidly selected. We conclude, at least for tufB, that forces other than gene dosage may be significant in selecting for chromosomal location