Reduction of Ribosome Level Triggers Flocculation of Fission Yeast Cells

Abstract

ABSTRACTDeletion of ribosomal protein L32 genes resulted in a nonsexual flocculation of fission yeast. Nonsexual flocculation also occurred when two other ribosomal protein genes,rpl21-2andrpl9-2, were deleted. However, deletion of two nonribosomal protein genes,mpgandfbp, did not cause flocculation. Overall transcript levels ofrpl32inrpl32-1Δ andrpl32-2Δ cells were reduced by 35.9% and 46.9%, respectively, and overall ribosome levels inrpl32-1Δ andrpl32-2Δ cells dropped 31.1% and 27.8%, respectively, compared to wild-type cells. Interestingly, ribosome protein expression levels and ribosome levels were also reduced greatly in sexually flocculating diploid YHL6381/WT (h+/h−) cells compared to a mixture of YHL6381 (h+) and WT (h−) nonflocculating haploid cells. Transcriptome analysis indicated that the reduction of ribosomal levels in sexual flocculating cells was caused by more-extensive suppression of ribosomal biosynthesis gene expression, while the reduction of ribosomal levels caused by deleting ribosomal protein genes in nonsexual flocculating cells was due to an imbalance between ribosomal proteins. We propose that once the reduction of ribosomal levels is below a certain threshold value, flocculation is triggered.