Sequence-specific RNA-binding protein that acts as a post-transcriptional repressor by binding the 3'-UTR of mRNA targets. Binds to an RNA consensus sequence, the Pumilio Response Element (PRE), 5'-UGUANAUA-3', that is related to the Nanos Response Element (NRE). Mediates post-transcriptional repression of transcripts via different mechanisms: acts via direct recruitment of the CCR4-POP2-NOT deadenylase leading to translational inhibition and mRNA degradation. Also mediates deadenylation-independent repression by promoting accessibility of miRNAs. Following growth factor stimulation, phosphorylated and binds to the 3'-UTR of CDKN1B/p27 mRNA, inducing a local conformational change that exposes miRNA-binding sites, promoting association of miR-221 and miR-222, efficient suppression of CDKN1B/p27 expression, and rapid entry to the cell cycle (By similarity). Acts as a post-transcriptional repressor of E2F3 mRNAs by binding to its 3'-UTR and facilitating miRNA regulation (By similarity). Represses a program of genes necessary to maintain genomic stability such as key mitotic, DNA repair and DNA replication factors. Its ability to repress those target mRNAs is regulated by the lncRNA NORAD (non-coding RNA activated by DNA damage) which, due to its high abundance and multitude of PUMILIO binding sites, is able to sequester a significant fraction of PUM1 and PUM2 in the cytoplasm (By similarity). Involved in neuronal functions by regulating ATXN1 mRNA levels: acts by binding to the 3'-UTR of ATXN1 transcripts, leading to their down-regulation independently of the miRNA machinery (PubMed:25768905). In testis, acts as a post-transcriptional regulator of spermatogenesis by binding to the 3'-UTR of mRNAs coding for regulators of p53/TP53 (PubMed:22342750). Involved in embryonic stem cell renewal by facilitating the exit from the ground state: acts by targeting mRNAs coding for naive pluripotency transcription factors and accelerates their down-regulation at the onset of differentiation (PubMed:24412312). Binds specifically to miRNA MIR199A precursor, with PUM2, regulates miRNA MIR199A expression at a postranscriptional level (By similarity). Recruits the CCR4-POP2-NOT deadenylase leading to translational inhibition and mRNA degradation (By similarity). Interacts with TRIM71 (via NHL repeats) in an RNA-dependent manner (By similarity). Widely expressed. Expressed in brain, heart, kidney, liver, lung, skin, intestine, spleen, testis and thymus. Weakly or not expressed in muscles and stomach. Expressed at various stages of myeloid and lymphoid cell development (PubMed:12667987). Highly expressed in testis (PubMed:22342750). Expressed in all major brain regions (at protein level) (PubMed:25768905). During the development of the testis, expressed 2 days postpartum (dpp) and then starts to increase at 14 dpp when pachytene spermatocytes first appear. The pumilio repeats mediate the association with RNA by packing together to form a right-handed superhelix that approximates a half donut. RNA-binding occurs on the concave side of the surface. PUM1 is composed of 8 pumilio repeats of 36 residues; each repeat binds a single nucleotide in its RNA target. Residues at positions 12 and 16 of the pumilio repeat bind each RNA base via hydrogen bonding or van der Waals contacts with the Watson-Crick edge, while the amino acid at position 13 makes a stacking interaction. The recognition of RNA by pumilio repeats is base specific: cysteine and glutamine at position 12 and 16, respectively, bind adenine; asparagine and glutamine bind uracil; and serine and glutamate bind guanine. Phosphorylation at Ser-715 promotes RNA-binding activity. Following growth factor stimulation phosphorylated at Ser-715, promoting binding to the 3'-UTR of CDKN1B/p27 mRNA. Mice are viable and grow to adulthood without apparent defects except that they are smaller than wild-type mice at 8 weeks of age (PubMed:22342750, PubMed:25768905). Males mice however show significantly reduced sperm counts and fertility: testicular hypoplasia is observed (PubMed:22342750). Heterozygous knockout mice manifest neurological dysfunction, hyperactivity, and progressive cerebellar signs including gross and fine motor incoordination (PubMed:25768905, PubMed:29474920). They show spontaneous seizures, abnormal EEG activity with generalized epileptiform spikes by the age of 16 weeks, and have smaller than normal cerebella (PubMed:29474920). Extended N-terminus.