How can a gene simultaneously transcribe and translate multiple proteins?
A single gene can be transcribed and translated into multiple different proteins primarily through gene expression regulation mechanisms, including alternative splicing, RNA editing, ribosomal frameshifting, internal ribosome entry site (IRES), polycistronic mRNA, among others. Here are some possible mechanisms:
I. Alternative Splicing
1. Mechanism
In eukaryotes, genes typically consist of exons (coding regions) and introns (non-coding regions). During the maturation of pre-mRNA, different combinations of exons can be spliced together to form different mRNAs, thus translating into different proteins.
2. Examples
(1) Dscam gene (fruit fly): Alternative splicing can produce more than 38,000 different proteins.
(2) Myoglobin/cardiac protein variants: The same gene can code for different muscle proteins in different tissues.
II. RNA Editing
1. Mechanism
In certain organisms, mRNA undergoes chemical modification after transcription, such as base changes (A→I, C→U), resulting in changes to the translated protein sequence.
2. Examples
APOB gene (humans): C→U editing in the intestine transforms ApoB100 into ApoB48, affecting lipoprotein metabolism.
III. Ribosomal Frameshifting
1. Mechanism
During translation, the ribosome may shift the reading frame (+1 or -1 frameshift) on the mRNA, resulting in different amino acid sequences and different protein products.
2. Examples
HIV-1 gag-pol gene: The virus uses a -1 frameshift mechanism to synthesize gag and pol proteins.
IV. Internal Ribosome Entry Site (IRES)
1. Mechanism
Some mRNAs contain IRES structures in the 5' UTR region, allowing ribosomes to bind directly to the mRNA and initiate translation without a 5' cap (cap-independent), producing different proteins from different start points.
2. Examples
Apoptosis-related genes c-myc, p53: Utilize IRES to regulate the synthesis of specific protein variants under stress conditions.
V. Polycistronic mRNA
1. Mechanism
In prokaryotes and the mitochondrial/chloroplast genomes of some eukaryotes, a single mRNA can encode multiple proteins, with each open reading frame (ORF) being translated into different proteins through ribosome binding sites (RBS).
2. Examples
E. coli lac operon: One mRNA can encode three proteins: LacZ, LacY, and LacA.
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