HU content and dynamics in Escherichia coli during the cell cycle and at different growth rates

DNA-binding proteins play an important role in maintaining bacterial chromosome structure and functions. Heat-unstable (HU) histone-like protein is one of the most abundant of these proteins and participates in all major chromosome-related activities. Owing to its low sequence specificity, HU fusions with fluorescent proteins were used for general staining of the nucleoid, aiming to reveal its morphology and dynamics. We have exploited a single chromosomal copy of hupA-egfp fusion under the native promoter and used quantitative microscopy imaging to investigate the amount and dynamics of HU alpha in Escherichia coli cells. We found that in steady-state growing populations the cellular HU alpha content is proportional to the cell size, whereas its concentration is size independent. Single-cell live microscopy imaging confirmed that the amount of HU alpha exponentially increases during the cell cycle, but its concentration is maintained constant. This supports the existence of an auto-regulatory mechanism underlying the HU alpha cellular level, in addition to reflecting the gene copy number. Both the HUa amount and concentration strongly increase with the cell growth rate in different culture media. Unexpectedly, the HU/DNA stoichiometry also remarkably increases with the growth rate. This last finding may be attributed to a higher requirement for maintaining the chromosome structure in nucleoids with higher complexity.