RNA-mediated gene regulation in Neisseria meningitidis and other nasopharyngeal pathogens

Sammanfattning: Neisseria meningitidis is a transient commensal bacterium of the upper respiratory tract and spreads through respiratory secretions produced by the carrier. While mostly harmless, the bacterium can cause invasive meningococcal disease by entering the circulatory system, leading to septicaemia and/or meningitis. It is widely theorised that the disease is the result of accidental dysfunctional gene regulation based on its genome plasticity. High mutation rates, genome re-arrangement and horizontal gene transfer may lead to a phenotype that is more immune evasive, more invasive, or better adapted to survive in the nasopharynx for an extended amount of time. Differential gene regulation is suggested to play a role in the ability of N. meningitidis to cause disease. This thesis presents several aspects of RNA-mediated gene regulation in N. meningitidis as an energy efficient mode of gene expression control. It is further demonstrated that Streptococcus pneumoniae and Haemophilus influenzae have developed a similar strategy to evade the immune system by sensing temperature. The temperature in the nasopharynx is slightly cooler than body temperature, which leads to adaption of colonising bacteria. Inflammation through immune responses can lead to higher temperature in that niche, serving as a warning signal for bacteria. Studies in this thesis unveiled, that S. pneumoniae and H. influenzae control production of their immune evasive polysaccharide capsule und factor H binding proteins by RNA thermosensors. Identifying the same ability in clinical isolates of S. pneumoniae suggests that the mode of temperature-based gene regulation is of advantage for nasopharyngeal bacteria and dysregulation may lead to invasive disease. In N. meningitidis, analysis of disease and carrier isolates uncovered a tandem repeat configuration in the RNA thermosensor that regulates its polysaccharide capsule production. Specific tandem repeat configurations induced higher capsulated phenotypes which were more associated with isolates that cause invasive disease. Other work in this thesis explored the ability of N meningitidis to scavenge nutrients in the nasopharynx. Two thiamine pyrophosphate dependent riboswitches were identified and linked to nutrient acquisition from dead bacteria. Salvaging thiamine from the environment and turning off de novo biosynthesis via the riboswitch can conserve energy and may facilitate colonisation. Furthermore, a novel cis-encoded RNA element has been identified to contain the Correia Repeat Enclosed Element (CREE), a repetitive sequence of the Neisseria genus genome. The work demonstrates that a CREE can be transcribed as the regulatory RNA Crrp. It interacts with other CREEs located on 5′-untranslated regions of mRNAs involved in type IV pilus biosynthesis, therefore influencing the natural competence and cell adhesion ability of N. meningitidis. In summary, this thesis demonstrates similarities in immune evasion control of nasopharyngeal pathogens and expands the knowledge about regulation of gene expression in N. meningitidis.