RocS drives chromosome segregation and nucleoid protection in Streptococcus pneumoniae.

Chromosome segregation in bacteria is poorly understood outside some prominent model strains <sup>1-5</sup> and even less is known about how it is coordinated with other cellular processes. This is the case for the opportunistic human pathogen Streptococcus pneumoniae (the pneumococcus) <sup>6</sup> , which lacks the Min and the nucleoid occlusion systems <sup>7</sup> , and possesses only an incomplete chromosome partitioning Par(A)BS system, in which ParA is absent <sup>8</sup> . The bacterial tyrosine kinase <sup>9</sup> CpsD, which is required for capsule production, was previously found to interfere with chromosome segregation <sup>10</sup> . Here, we identify a protein of unknown function that interacts with CpsD and drives chromosome segregation. RocS (Regulator of Chromosome Segregation) is a membrane-bound protein that interacts with both DNA and the chromosome partitioning protein ParB to properly segregate the origin of replication region to new daughter cells. In addition, we show that RocS interacts with the cell division protein FtsZ and hinders cell division. Altogether, this work reveals that RocS is the cornerstone of a nucleoid protection system ensuring proper chromosome segregation and cell division in coordination with the biogenesis of the protective capsular layer