Because microRNAs (miRNAs) are critical regulators of key processes such as development and differentiation, their expression levels must be precisely regulated. Recent progress has been made in understanding regulation of miRNA transcription and multiple stages of biogenesis (i.e., processing) 
. However, the regulation of miRNA degradation is poorly understood in humans. Most studies describe miRNAs as being remarkably stable, with reported half-lives for human miRNAs ranging from hours to several days 
. In plants and worms, exonucleases that degrade miRNAs have been identified 
, but in humans no global mechanisms for regulating the turnover of miRNAs have been described.
We and others have observed that mature miRNAs show a diversity of nucleotide additions at their 3′ end in metazoan miRNA high-throughput sequencing studies 
. These 3′ additions have recently been demonstrated to be a conserved, biological phenomenon in humans, mice, plants, and flies 
. The origins of miRNA 3′ additions can be diverse, and include both aberrant enzymatic processing as well as post-transcriptional modification of precursor and mature miRNAs 
. MicroRNA variants, commonly referred to as “isomiRs”, substantially expand the known miRNA transcriptome; however, the biological significance of 3′ additions is not yet well-understood. In plants, methylation and adenylation rendered some miRNAs resistant to degradation 
. In contrast, uridylation by nucleotidyl transferase enzymes led to increased degradation of specific miRNAs in both plants and algae 
. Taken together, these studies demonstrate that the modification of a miRNA with a single nucleotide addition can be sufficient for altering miRNA decay.
In humans, the role of 3′ additions is poorly understood, as only a handful of reports have examined the effects of A and U additions on the stability of a small number of miRNAs. The addition of a 3′ A to the miRNA miR-122 was associated with increased abundance of the miRNA 
. On the other hand, the addition of a 3′ U to miR-26a did not change the abundance of the miRNA, but instead prevented the miRNA from repressing its mRNA target 
. Recent work has suggested that 3′ additions may be involved in miRNA turnover, as increased uridylation of an exogenously expressed miRNA has been observed during its decay 
. MicroRNA isomiRs are found in association with the Ago proteins, although nucleotide additions were shown to reduce miRNA association with certain Ago proteins in humans 
. Therefore, 3′ additions have been hypothesized to have miRNA-specific effects on controlling either the targeting or stability of a miRNA.
In humans, many miRNAs feature 3′ non-templated additions of an adenosine (A) or uridine (U), which do not match the genomic or precursor sequences 
, and thus are not ambiguous in origin. Non-templated 3′ additions are regulated by multiple members of the nucleotidyl transferase family in humans. We and others have found that the PAPD4 enzyme governs the 3′ A additions of a broad panel of miRNAs 
. We also identified multiple other enzymes, including TUT1, PAPD5, MTPAP, ZCCHC11, and ZCCHC6, that regulate 3′ additions in a miRNA-specific manner 
. In order to investigate the biological significance of miRNA 3′ additions, here we examined the relationship between changes in miRNA additions and miRNA abundance following suppression of each of these nucleotidyl transferases. We found that among the miRNAs we examined, those with increased 3′ addition–particularly of a 3′ U–were associated with decreased miRNA abundance. We also identified a nucleotidyl transferase, TUT1, which exerts broad effects on miRNA expression levels despite its seemingly restrictive regulation of 3′ additions to specific miRNAs. This represents a new role for the TUT1 nucleotidyl transferase, which has previously been shown to regulate the stability of a subset of mRNA transcripts and the U6 snRNA 
. Taken together, our studies demonstrate that TUT1 serves as not only an important modulator of 3′ additions to specific miRNAs, but also more globally as a post-transcriptional regulator of the abundance of a large number of miRNAs through an indirect mechanism.