Final answer:
In group I self-splicing, the guanidine is in the form of a guanidinium ion, which serves as an analogue of the arginine side chain and inhibits splicing by competing with the necessary guanosine nucleotide at the active site.
Step-by-step explanation:
In group I self-splicing, the guanidine is in the form of a guanidinium ion. This guanidinium ion acts as an analogue of the arginine side chain and is bound within the active site for the co-splicing substrate. Specifically, it interacts at the site of a conserved arginine codon within the RNA. However, because the guanidinium ion competes with the guanosine nucleotide necessary for splicing at this site, it inhibits the splicing process.
The natural cases of self-splicing involve the Tetrahymena self-splicing group I intron which binds arginine and its analogue guanidinium ion, reflecting a significant overlap with the RNA structures that were selected to bind arginine. Similarly, riboswitches regulated by guanidinium ion in bacteria demonstrate the biological use of the affinity between arginine/guanidinium and coding triplets, carrying this chemical connection into contemporary organisms.