A number of examples are now known, where a protein or RNA regulates its own production. Several proteins work as repressors for repression of their own production (Table 35.2). This is achieved by binding of repressor protein to ribosome binding site (or
Shine-Delgarno sequence), or initiating codon (AUG) of mRNA.
An important example of autogenous regulation is p32 protein, which is coded by T4 gene 32 and needed for recombination, repair and replication of DNA. Whenever this protein p32 is in excess, it binds to mRNA and prevents initiation of translation. Several genes for ribosomal proteins, protein synthesis factors and RNA polymerase subunits (which are organized in a few operons), are also autogenously regulated at the translation level. In these cases, a regulatory protein binds to a polycistronic mRNA and inhibits expression of a contiguous sets of genes within an operon, always including its own gene. This is true about ribosomal proteins (r proteins). These r proteins are generally utilized for binding with rRNA for ribosomes assembly, but if there is excess of r proteins, an r protein may bind to polycistronic mRNA and inhibit synthesis of r proteins.
In the above cases, the mRNA remains intact but can not be translated. However, there are other systems, where mRNA may be degraded as in case of synthesis of
tubulin in eukaryotic cells. In this case tubulin molecules are normally assembled as microtubules, but when present in excess these may bind at a short specific sequence of its mRNA causing its degradation.
