Inteins: The internal fragments of precursor proteins (INternal proTEINS) that are autocatalytically removed by PROTEIN SPLICING. The flanking fragments (EXTEINS) are ligated forming mature proteins. The nucleic acid sequences coding for inteins are considered to be MOBILE GENETIC ELEMENTS. Inteins are composed of self-splicing domains and an endonuclease domain which plays a role in the spread of the intein's genomic sequence. Mini-inteins are composed of the self-splicing domains only.Protein Splicing: The excision of in-frame internal protein sequences (INTEINS) of a precursor protein, coupled with ligation of the flanking sequences (EXTEINS). Protein splicing is an autocatalytic reaction and results in the production of two proteins from a single primary translation product: the intein and the mature protein.Exteins: The (EXternal proTEIN) sequences flanking INTEINS, which are internal fragments of precursor proteins removed by PROTEIN SPLICING. Concomitantly, as the inteins are excised, the exteins are ligated to form mature proteins.Trans-Splicing: The joining of RNA from two different genes. One type of trans-splicing is the "spliced leader" type (primarily found in protozoans such as trypanosomes and in lower invertebrates such as nematodes) which results in the addition of a capped, noncoding, spliced leader sequence to the 5' end of mRNAs. Another type of trans-splicing is the "discontinuous group II introns" type (found in plant/algal chloroplasts and plant mitochondria) which results in the joining of two independently transcribed coding sequences. Both are mechanistically similar to conventional nuclear pre-mRNA cis-splicing. Mammalian cells are also capable of trans-splicing.DnaB Helicases: A family of DNA helicases that participate in DNA REPLICATION. They assemble into hexameric rings with a central channel and unwind DNA processively in the 5' to 3' direction. DnaB helicases are considered the primary replicative helicases for most prokaryotic organisms.Nostoc: A form-genus of CYANOBACTERIA in the order Nostocales. Trichomes composed of spherical or ovoid vegetative cells along with heterocysts and akinetes. The species form symbiotic associations with a wide range of eukaryotes.Mycobacteriophages: Viruses whose host is one or more Mycobacterium species. They include both temperate and virulent types.Protein Engineering: Procedures by which protein structure and function are changed or created in vitro by altering existing or synthesizing new structural genes that direct the synthesis of proteins with sought-after properties. Such procedures may include the design of MOLECULAR MODELS of proteins using COMPUTER GRAPHICS or other molecular modeling techniques; site-specific mutagenesis (MUTAGENESIS, SITE-SPECIFIC) of existing genes; and DIRECTED MOLECULAR EVOLUTION techniques to create new genes.Methanococcus: A genus of anaerobic coccoid METHANOCOCCACEAE whose organisms are motile by means of polar tufts of flagella. These methanogens are found in salt marshes, marine and estuarine sediments, and the intestinal tract of animals.