Retrotransposons are mobile genetic elements that replicate via an RNA intermediate and are similar to retroviruses. The long terminal repeat retrotransposon Ty1 is the most abundant transposon in the widely used model organism Saccharomyces cerevisiae. Ty1 contains two genes; GAG, which encodes the capsid and POL, which encodes protease (PR), integrase (IN), and reverse transcriptase (RT). Retrotransposon movement impacts the host genome by insertional mutagenesis or longer-range effects on host gene expression. Chromosomal rearrangements can also occur by recombination between retrotransposons. Host organisms have evolved different ways to control the level of retrotransposition such as DNA methylation and RNA interference, but those pathways are absent in S. cerevisiae. For Ty1, a novel copy number control (CNC) mechanism blocks Ty1 movement by inhibiting virus-like particle (VLP) assembly, function and protein processing. Interestingly, CNC is mediated by the self-encoded restriction factor p22, which is derived from GAG and encoded by Ty1 internally-initiated (Ty1i) RNA. While p22 is a key player in CNC, cellular genes or environmental factors that affect p22 have not been studied. We hypothesized that a subset of Ty1 cofactors affect CNC, and these genes can be identified by CNC-specific secondary screens of Ty1 cofactor mutants. We identified rRNA processing/ribosome assembly factor (loc1) and ribosomal subunit proteins (rps0b and rpl7a) that exhibit a severe decrease in Ty1 mobility in strains undergoing CNC. Further examination revealed that the level of p22 increases in these mutants. Modest increases in the level of Ty1i RNA relative to Ty1 mRNA and the half-life of p22 also affect CNC in a loc1 mutant. Furthermore, a loc1 mutant exhibits defects in Gag multimerization, Gag complex assembly, and packaging of Ty1 mRNA in VLPs. Strikingly, cytoplasmic foci containing Ty1 products that are sites for VLP assembly are abolished in a loc1 mutant. In summary, my work emphasizes the importance of ribosome biogenesis in mediating host control of Ty1, and may identify a novel stress response that alters VLP assembly.