Deconstructing the structural conservation of distantly related bacterial nucleoid-associated proteins using functional chimeras

Nucleoid-associated proteins (NAPs) are DNA-binding proteins critical for the organization and function of the bacterial chromosome. A subclass of NAPs, including Caulobacter crescentus GapR and Escherichia coli H-NS, preferentially bind AT-rich regions of the nucleoid, but phylogenetic groups that encode GapR rarely encode H-NS. Here, utilizing genetic, biochemical, and biophysical studies of GapR in light of a recent DNA-bound crystal structure of GapR (Guo et al, 2018), we show that although evolutionarily distant, GapR and H-NS possess two regions that are structurally and functionally conserved. These regions are involved in self-association and DNA-binding, even though the two proteins oligomerize and regulate transcription differently. Functional analysis of GapR and H-NS protein chimeras identified structural elements present in H-NS but absent in GapR that rationalize differences in transcriptional regulation. In addition, we identified a sequence element unique to GapR that enables assembly into its tetrameric state. Using fluid-atomic force microscopy, we showed that GapR is capable of bridging DNA molecules in vitro. Together, these results demonstrate that two distantly related NAPs utilize evolutionarily conserved structural elements to serve specialized cellular roles via distinct mechanisms.