In the study of bacteria, a longstanding dogma held that two molecular machines — RNA polymerase and ribosomes — worked so closely in tandem that they were effectively attached. This close coupling of transcription and translation was thought to be fundamental to gene expression, partly because the trailing ribosome could shield nascent gene products from a quality-control protein called Rho.
However, in bacteria exhibiting runaway transcription, such as Bacillus subtilis, the polymerase speeds ahead, unhitched from its protective ribosome. Surprisingly, Rho targeted primarily noncoding, useless RNA products in these bacteria. Recent research from the Department of Biology reveals that the secret to Rho’s quality-control specificity lies in the nucleotide sequences of coding DNA strands. Julia Dierksheide, a graduate student in the Li Lab, stated, “We hypothesized that Rho was regulated by sequence, but the fact that the sequence alone could protect any gene in the entire B. subtilis genome from Rho was really surprising.”
Rho serves as a termination factor, preventing bacteria from wasting resources on futile RNA transcripts. All the information a bacterial cell needs is encoded in its DNA, which consists of two strands of nucleic acids forming a double helix, with genetic information codified in base pairs: purines guanine and adenine matched with pyrimidines cytosine and thymine.
Coding DNA strands in certain bacteria are known to be significantly higher in purines compared to the rest of the genome. The researchers found that this purine bias alone shields productive mRNA transcripts from Rho-mediated termination. Dierksheide remarked, “I enjoy having a big, complicated dataset and trying to reduce it to biological meaning.”
Bacterial species that have lost Rho over generations no longer exhibit this strong purine bias. Rho also regulates bacteria's motility, biofilm formation, and sporulation, which are critical for survival. The purine bias could provide protection against foreign DNA insertion during viral bacteriophage infections. Dierksheide emphasized, “Bacteria exist as single cells, so everything they do, they must do through gene expression. Understanding the fundamental details about gene expression and how a cell encodes all the information it needs to survive in the nucleotide sequence of the genome is really exciting.”
Although the exact mechanism underlying Rho’s specificity remains unclear, these results reveal an underlying code in the composition of bacterial genomes. Dierksheide hopes to perform a similar screen to characterize Rho’s specificity in Escherichia coli, which diverged from B. subtilis on the evolutionary tree an estimated 2 billion years ago and still exhibits coupled transcription-translation. A systematic comparison to E. coli Rho could help reveal how this heightened stringency arose.
This information will be critical for engineering diverse bacterial species for applications including the production of therapeutic agents. Bacterial species like B. subtilis may be better models for this process because they have abundant secretion pathways, making it easier to produce and isolate proteins in large quantities. Gene-Wei Li, the lead author of the study, stated, “Our findings reveal an important criterion for successful sequence design that must be considered in expression engineering. There are so many cryptic messages in the genome, like the purine bias, and we are just beginning to decipher what they mean.”
Blogger's Review: This study uncovers the underlying mechanisms of purine bias in bacterial genomes, emphasizing how sequence characteristics in gene expression influence bacterial survival strategies. It provides new insights into gene regulation in bacteria and opens avenues for future applications in genetic engineering, warranting further exploration.