| Summary: | <i>Streptococcus pneumoniae</i> is a major cause of morbidity and mortality worldwide, and about 30% of the pneumococcal clinical isolates show type I pili-like structures. These long proteinaceous polymers extending from the bacterial surface are encoded by pilus islet 1 and play major roles in adhesion and host colonization. Pili expression is bistable and is controlled by the transcriptional activator RlrA. In this work, we demonstrate that the previously identified small noncoding RNA <i>srn135</i> also participates in pilus regulation. Our findings show that <i>srn135</i> is generated upon processing of the 5′-UTR region of <i>rrgA</i> messenger and its deletion prevents the synthesis of RrgA, the main pili adhesin. Moreover, overexpression of <i>srn135</i> increases the expression of all pili genes and rises the percentage of piliated bacteria within a clonal population. This regulation is mediated by the stabilization of <i>rlrA</i> mRNA since higher levels of <i>srn135</i> increase its half-life to 165%. Our findings suggest that <i>srn135</i> has a dual role in pilus expression acting both in <i>cis-</i> (on the RrgA levels) and in <i>trans-</i> (modulating the levels of RlrA) and contributes to the delicate balance between pili expressing and non-expressing bacteria.
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