Table 12. Some inclusions in bacterial cells. Cytoplasmic inclusions Where found Composition Function glycogen many bacteria . E. coli polyglucose reserve carbon and energy source polybetahydroxybutyric acid (PHB) many bacteria . Pseudomonas polymerized hydroxy butyrate reserve carbon and energy source polyphosphate (volutin granules) many bacteria . Corynebacterium linear or cyclical polymers of PO4 reserve phosphate; possibly a reserve of high energy phosphate sulfur globules phototrophic purple and green sulfur bacteria and lithotrophic colorless sulfur bacteria elemental sulfur reserve of electrons (reducing source) in phototrophs; reserve energy source in lithotrophs gas vesicles aquatic bacteria especially cyanobacteria protein hulls or shells inflated with gases buoyancy (floatation) in the vertical water column parasporal crystals endospore-forming bacilli (genus Bacillus ) protein unknown but toxic to certain insects magnetosomes certain aquatic bacteria magnetite (iron oxide) Fe3O4 orienting and migrating along geo- magnetic field lines carboxysomes many autotrophic bacteria enzymes for autotrophic CO2 fixation site of CO2 fixation phycobilisomes cyanobacteria phycobiliproteins light-harvesting pigments chlorosomes Green bacteria lipid and protein and bacteriochlorophyll light-harvesting pigments and antennae
Ribosomes are the workplaces of protein biosynthesis , the process of translating mRNA into protein . The mRNA comprises a series of codons that dictate to the ribosome the sequence of the amino acids needed to make the protein. Using the mRNA as a template, the ribosome traverses each codon (3 nucleotides) of the mRNA, pairing it with the appropriate amino acid provided by an aminoacyl-tRNA . Aminoacyl-tRNA contains a complementary anticodon on one end and the appropriate amino acid on the other. For fast and accurate recognition of the appropriate tRNA, the ribosome utilizes large conformational changes ( conformational proofreading ) .  The small ribosomal subunit, typically bound to an aminoacyl-tRNA containing the amino acid methionine , binds to an AUG codon on the mRNA and recruits the large ribosomal subunit. The ribosome contains three RNA binding sites, designated A, P and E. The A site binds an aminoacyl-tRNA; the P site binds a peptidyl-tRNA (a tRNA bound to the peptide being synthesized); and the E site binds a free tRNA before it exits the ribosome. Protein synthesis begins at a start codon AUG near the 5' end of the mRNA. mRNA binds to the P site of the ribosome first. The ribosome is able to identify the start codon by use of the Shine-Dalgarno sequence of the mRNA in prokaryotes and Kozak box in eukaryotes.