Introns and exons are nucleotide sequences within a gene. Exons are “expressed sequences” containing coding information for protein production. Introns are “intervening sequences,” which are non-coding regions located between exons that are removed during gene expression. Generally, prokaryotes—including bacteria and archaea—do not possess this typical intron/exon gene structure, although some rare exceptions exist.
Understanding Split Genes in Eukaryotes
The genes of eukaryotes, such as plants, animals, and fungi, are characterized by a “split gene” architecture, where coding exons are interrupted by non-coding introns. When a eukaryotic gene is transcribed from DNA into a precursor messenger RNA (pre-mRNA), it contains both the exons and the introns. This pre-mRNA must undergo a complex post-transcriptional modification called RNA splicing to become a mature messenger RNA (mRNA).
During splicing, the introns are precisely excised, and the flanking exons are joined together to create a continuous coding sequence. This process is carried out by the spliceosome, a large molecular complex unique to eukaryotes. The separation of the nucleus (where splicing occurs) from the cytoplasm (where translation takes place) provides the necessary time and space for this processing.
The split gene structure allows for alternative splicing. This process enables a single gene to produce multiple distinct protein variants by selecting which exons to include in the final mature mRNA. This mechanism increases the coding capacity of the genome, allowing complex organisms to generate a vast diversity of proteins.
The Continuous Structure of Prokaryotic Genes
In stark contrast to the eukaryotic model, the genes of most prokaryotes are organized as continuous coding sequences that lack introns. The DNA sequence transcribed into mRNA is immediately ready for translation, meaning the genetic message is uninterrupted. This streamlined organization allows for high efficiency and rapid gene expression, which is advantageous for organisms that need to reproduce quickly.
A defining feature of prokaryotic gene organization is the operon, where multiple genes that encode functionally related proteins are clustered together and transcribed from a single promoter. The resulting transcript is called a polycistronic mRNA because it contains the coding information for several different proteins. This structure allows the cell to regulate the production of all necessary components for a specific pathway, such as lactose metabolism, simultaneously.
The speed of gene expression in prokaryotes is facilitated by coupled transcription and translation. Since prokaryotic cells lack a nucleus, the ribosome can attach to the mRNA and begin translation while the RNA polymerase is still transcribing the DNA. This simultaneous process would be impossible if the transcript required time-consuming splicing to remove intervening sequences.
Intron-Like Elements and Inteins in Bacteria
While the typical spliceosomal introns of eukaryotes are absent, some prokaryotes possess analogous intervening sequences. These elements are primarily self-splicing introns, classified as Group I and Group II, found in the genes for ribosomal RNA (rRNA) and transfer RNA (tRNA) in various bacteria and archaea. Unlike eukaryotic introns, these self-splicing introns are ribozymes, meaning the RNA molecule performs the catalytic action to excise the intron and join the exons without requiring a large protein complex.
Group I introns are excised via transesterification reactions initiated by an external guanosine cofactor. Group II introns are considered evolutionarily significant because their splicing mechanism is structurally similar to that of the spliceosomal introns found in the eukaryotic nucleus. These prokaryotic introns are often viewed as mobile genetic elements that have inserted themselves into the host genome.
Another distinct type of intervening sequence found in prokaryotes is the intein, which operates at the protein level, not the RNA level. Inteins are segments of a polypeptide chain that are autocatalytically removed from the protein after translation is complete. This process, called protein splicing, involves the intein excising itself and joining the two flanking protein segments (exteins) together.