Enhanced binding of an HU homologue under increased DNA supercoiling preserves chromosome organisation and sustains Streptomyces hyphal growth

Bacterial chromosome topology is controlled by topoisomerases and nucleoid-associated proteins (NAPs). While topoisomerases regulate DNA supercoiling, NAPs introduce bends or coat DNA upon its binding, affecting DNA loop formation. Streptomyces, hyphal, multigenomic bacteria known for producing numerous clinically important compounds, use the highly processive topoisomerase I (TopA) to remove excessive negative DNA supercoils. Elongated vegetative Streptomyces cells contain multiple copies of their linear chromosome, which remain relaxed and relatively evenly distributed. Here, we explored how TopA cooperates with HupA, an HU homologue that is the most abundant Streptomyces NAP. We verified that HupA has an increased affinity for supercoiled DNA in vivo and in vitro. Analysis of mutant strains demonstrated that HupA elimination is detrimental under high DNA supercoiling conditions. The absence of HupA, combined with decreased TopA levels, disrupted chromosome distribution in hyphal cells, eventually inhibiting hyphal growth. We concluded that increased HupA binding to DNA under elevated chromosome supercoiling conditions is critical for the preservation of chromosome organisation.