Department of Biology, Indiana University, Bloomington, Indiana, USA.
J Bacteriol. 2022 Aug 16;204(8):e0011922. doi: 10.1128/jb.00119-22. Epub 2022 May 12.
Nucleoid-associated proteins (NAPs) help structure bacterial genomes and function in an array of DNA transactions, including transcription, recombination, and repair. In most bacteria, NAPs are nonessential in part due to functional redundancy. In contrast, in Bacillus subtilis the HU homolog HBsu is essential for cell viability. HBsu helps compact the B. subtilis chromosome and participates in homologous recombination and DNA repair. However, none of these activities explain HBsu's essentiality. Here, using two complementary conditional HBsu alleles, we investigated the terminal phenotype of the mutants. Our analysis revealed that cells without functional HBsu fail to initiate DNA replication. Importantly, when the chromosomal replication origin () was replaced with a plasmid origin () whose replication does not require the initiator DnaA, cells without HBsu initiated DNA replication normally. However, HBsu was still essential in this -containing strain. We conclude that HBsu plays an essential role in the initiation of DNA replication, likely acting to promote origin melting by DnaA, but also has a second essential function that remains to be discovered. While it is common for a bacterial species to express multiple nucleoid-associated proteins (NAPs), NAPs are seldomly essential for cell survival. In B. subtilis, HBsu is a NAP essential for cell viability. Here, using conditional alleles to rapidly remove or inactivate HBsu, we show that the absence of HBsu abolishes the initiation of DNA replication . Understanding HBsu's function can provide new insights into the regulation of DNA replication initiation in bacteria.
类核相关蛋白（NAPs）有助于构建细菌基因组，并在转录、重组和修复等多种 DNA 代谢中发挥作用。在大多数细菌中，NAPs 的部分功能是冗余的，因此它们是非必需的。相比之下，枯草芽孢杆菌的 HU 同源物 HBsu 对细胞活力是必需的。HBsu 有助于压缩枯草芽孢杆菌的染色体，并参与同源重组和 DNA 修复。然而，这些活动都无法解释 HBsu 的必需性。在这里，我们使用两个互补的条件性 HBsu 等位基因，研究了突变体的末端表型。分析表明，没有功能性 HBsu 的细胞无法启动 DNA 复制。重要的是，当染色体复制起点（oriC）被一个质粒复制起点（ori）取代时，其复制不需要起始蛋白 DnaA，没有 HBsu 的细胞可以正常启动 DNA 复制。然而，在含有这种质粒的菌株中，HBsu 仍然是必需的。我们的结论是，HBsu 在 DNA 复制的起始中发挥着重要作用，可能通过 DnaA 促进起始点的融化，但它还有第二个尚未发现的必需功能。虽然一个细菌物种表达多种类核相关蛋白（NAPs）很常见，但 NAPs 很少对细胞存活是必需的。在枯草芽孢杆菌中，HBsu 是一种对细胞活力必需的 NAP。在这里，我们使用条件性等位基因快速去除或失活 HBsu，结果表明 HBsu 的缺失会阻止 DNA 复制的起始。了解 HBsu 的功能可以为细菌中 DNA 复制起始的调控提供新的见解。
