Post-transcriptional regulation of cyclin A and B mRNAs by Bruno 1, Cup, and P-bodies.

Cell cycle progression relies on tightly regulated Cyclin synthesis and degradation, with Cyclins A and B activating CDK1 to drive mitosis. Dysregulation of Cyclin levels is linked to tumorigenesis, underscoring the importance of studying cyclin mRNA control for cancer therapy development. Using super-resolution microscopy, we show that cyclin A and cyclin B mRNAs associate with Bruno 1 and Cup in nurse cells, and that depletion of either protein disrupts Cyclin expression and reduces mRNA levels. Both mRNAs also accumulate in Me31B-marked P-bodies; however, Me31B selectively affects cyclin B, causing its stage-specific de-repression and decreased stability, while cyclin A remains unaffected. Loss of Me31B enhances cyclin B mRNA's association with Cup, suggesting P-body-independent repression mechanisms. These results highlight the nuanced, mRNA-specific roles of P-body condensates in post-transcriptional regulation, challenging the idea of a uniform, binary mechanism of mRNA repression in P-bodies.