What is the Difference Between snRNA and snoRNA

The key difference between snRNA and snoRNA is that snRNA involves the alternative splicing of pre-mRNA while snoRNA involves modifying rRNA and tRNA, mRNA editing, and genome imprinting.

Small RNA are polymeric RNA molecules that consist of less than 200 nucleotides. They are usually non-coding. They exist among messenger RNAs to carry signals. Small RNAs originate from perfect double-stranded RNA, which is produced by the action of RNA-dependant RNA polymerase. Small RNAs play an important role in cell differentiation, growth and proliferation, apoptosis, metabolism, migration, and defense. Therefore, small RNAs are important and critical regulators of development and physiology. Small nuclear RNA and small nucleolar RNA are two classes of small RNA molecules.

CONTENTS

1. Overview and Key Difference
2. What is snRNA  
3. What is snoRNA
4. Similarities – snRNA and snoRNA
5. snRNA vs snoRNA in Tabular Form
6. Summary – snRNA vs snoRNA

What is snRNA?

Small nuclear RNA or snRNA is a class of small RNA molecules found in the nucleus of eukaryotic cells. The average length of an snRNA is approximately 150 nucleotides. RNA polymerase II or RNA polymerase III transcribe snRNA. The main function of snRNA is the processing of pre-messenger RNA in the nucleus. They also help to regulate transcription factors or RNA polymerase II and maintain telomeres.

There are two classes of snRNA based on common sequence features and associated protein factors such as RNA binding LSm protein. The two classes are Sm-class snRNA and Lsm-class snRNA. Sm-class snRNA consists of high uridine contents of U1, U2, U4, U4atac, U5, U7, U11, and U12. RNA polymerase II transcribes SM-class snRNA. After the transcription of pre-snRNA, they usually receive a 7-methylguanosine 5’ cap in the nucleus. Then they are exported to the cytoplasm through nuclear pores for further processing. Lsm-class snRNA has a high uridine content of U6 and U6atac. RNA polymerase III transcribes Lsm-class snRNA, and it does not leave the nucleus. The most common human snRNA components are U1 spliceosomal RNA, U2 spliceosomal RNA, U4 spliceosomal RNA, U5 spliceosomal RNA, and U6 spliceosomal RNA.

What is snoRNA?

Small nucleolar RNA or snoRNA is a class of small RNA molecules that guide chemical modifications in other RNA such as ribosomal RNA, transfer RNA, and small nuclear RNA. Each snoRNA molecule is associated with around four core proteins in RNA/protein complex during the modification process. The snoRNA contains an antisense element which is around 10-20 nucleotides. These bases are complimentary to the sequence surrounding the nucleotides that target the modification in the pre-RNA molecule.

There are two classes of snoRNAs. They are the C/D box snoRNA and H/ACA box snoRNA. C/D box snoRNA is associated with methylation, and H/ACA box snoRNA is associated with pseudo-uridylation. Each snoRNA molecule acts as a guide for one or two modifications in a target RNA. C/D box snoRNA contains two short conserved sequence motifs C and D, near the 5’ and 3’ ends of the snoRNA, respectively. Short regions consisting of 5 nucleotides arrange upstream of the C box and downstream of the D box and form a stem-box structure. This brings the C and D box motifs to close proximity. Stem-box structure is important for correct snoRNA synthesis and nucleolar localization. H/ACA box snoRNA has a secondary structure that consists of two hairpins and two single-stranded regions. This is commonly known as the hairpin-hinge-hairpin-tail structure. H/ACA box snoRNA also consists of two conserved motifs H and ACA. Both are located in single-stranded regions. The H box is in the hinge, and ACA is in the tail region. Three nucleotides form the 3’ end of the sequence.

What are the Similarities Between snRNA and snoRNA?

• snRNA and snoRNA are small RNAs.
• Both are present in eukaryotic cells.
• Both are non-coding RNA molecules.
• Moreover, they participate in modifying RNA during the transcription process.

What is the Difference Between snRNA and snoRNA?

snRNA participates in alternative splicing of pre-mRNA while snoRNA mainly modifies RNA molecules. Thus, this is the key difference between snRNA and snoRNA. Furthermore, snRNAs are about 150 nucleotides in length and are often found linked with a group of proteins and complexes called small nuclear ribonucleoproteins. snoRNAs are about 60-170 nucleotides in length and are mainly found in the nucleolus.

Moreover, snRNAs are transcribed by either RNA polymerase II or RNA polymerase III whereas, snoRNA is transcribed only by RNA polymerase II.

The below infographic presents the differences between snRNA and snoRNA in tabular form for side by side comparison.

Summary – snRNA vs snoRNA

snRNA and snoRNA are classes of small RNA molecules. snRNA participates in alternative splicing of pre-mRNA while snoRNA mainly modifies RNA molecules. Thus, this is the key difference between snRNA and snoRNA. Small RNA are polymeric RNA molecules that consist of less than 200 nucleotides in length and are usually non-coding. snRNAs are found in the nucleus of eukaryotes. snoRNAs are found in archaea and eukaryotes. Transcription of snRNA takes place through RNA polymerase II and II, while only RNA polymerase II is involved in transcribing snoRNA. So, this summarizes the difference between snRNA and snoRNA.

Reference:

1. “Small Nuclear RNA.” An Overview | ScienceDirect Topics.
2. “Small Nucleolar RNA.” An Overview | ScienceDirect Topics.

Image Courtesy:

1. “Bacterial small rna mechanisms” By Rz537816 – Rachel L. Zapf – Own work (CC BY-SA 4.0) via Commons Wikimedia
2. “RF02786” By Rfam – Source Rfam database and (CC0) via Commons Wikimedia