Microbiology and Immunology

Ribosomes WORLD OF MICROBIOLOGY AND IMMUNOLOGY

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Ribosomal RNAs fold in very complex ways. Their

structure is an important clue to the evolutionary relationships

found between different kinds of organisms. Sequence com-

parisons of the various rRNAs across various species show

that even though their base sequences vary widely, evolution

has conserved their secondary structures, therefore, organiza-

tion must be important for their function.

Since the 1970s, nucleic acids’ cellular processes have

become the basis for genetic engineering, in which scientists

add or remove genes in order to alter the characteristics or

behavior of cells. Such techniques are used in agriculture,

pharmaceutical and other chemical manufacturing, and med-

ical treatments for cancer and other diseases.

See alsoBiochemistry; Genetic regulation of eukaryotic cells

RRibosomesIBOSOMES

Ribosomes are organelles that play a key role in the manufac-

ture of proteins. Found throughout the cell, ribosomes are

composed of ribosomal ribonucleic acid(rRNA) and proteins.

They are the sites of protein synthesis.

Although Robert Hookefirst used a light microscopeto

look at cells in 1665, it was only during the last few decades

that the cell’s organelles were discovered. This is primarily

because light microscopes do not have the magnifying power

required to see these tiny structures. Using an electron micro-

scope, scientists have been able to see most of the cells sub-

structures, including the ribosomes.

Ribosomes are composed of a variety of proteins and

rRNA. They are organized in two functional subunits that are

constructed in the cell’s nucleolus. One is a small subunit that

has a squashed shape, while another is a large subunit that is

spherical in shape. The large subunit is about twice as big as

the small unit. The subunits usually exist separately, but join

when they are attached to a messenger RNA(mRNA). This ini-

tiates protein synthesis.

Production of a protein begins with initiation. In this

step, the ribosomal small subunit binds to the mRNA along

with the first transfer RNA (tRNA). The next step is elonga-

tion, where the ribosome moves along the mRNA and strings

together the amino acids one by one. Finally, the ribosome

encounters a stop sequence and the two subunits release the

mRNA, the polypeptide chain, and the tRNA.

Protein synthesis occurs at specific sites within the ribo-

some. The P site of a ribosome contains the growing protein

chain. The A site holds the tRNA that has the next amino acid.

The two sites are held close together and a chemical reaction

occurs. When the stop signal is present on the mRNA, protein

synthesis halts. The polypeptide chain is released and the ribo-

some subunits are returned to the pool of ribosome units in the

cytoplasm.

Ribosomes are found in two locations in the cell. Free

ribosomes are dispersed throughout the cytoplasm. Bound

ribosomes are attached to a membranous structure called the

endoplasmic reticulum. Most cell proteins are made by the

free ribosomes. Bound ribosomes are instrumental in produc-

ing proteins that function within or across the cell membrane.

Depending on the cell type, there can be as many as a few mil-

lion ribosomes in a single cell.

Because most cells contain a large number of ribo-

somes, rRNA is the most abundant type of RNA. rRNA plays

an active role in ribosome function. It interacts with both the

mRNA and tRNA and helps maintain the necessary structure.

Transfer RNA is the molecule that interacts with the mRNA

during protein synthesis and is able to read a three amino acid

sequence. On the opposite end of the tRNAs, amino acids are

bonded on a growing polypeptide chain. Generally, it takes

about a minute for a single ribosome to make an average sized

protein. However, several ribosomes can work on a single

mRNA at the same time. This allows the cell to make many

copies of a single protein rapidly. Sometimes these multiple

ribosomes, or polysomes, can become so large that they can be

seen with a light microscope.

The ribosomes in eukaryotes and prokaryotes are

slightly different. Eukaryotic ribosomes are generally larger

and are made up of more proteins. Since many diseases are

caused by prokaryotes, these slight differences have important

medical implications. Drugs have been developed that can

inhibit the function of a prokaryotic ribosome, but leave the

eukaryotic ribosome unaffected. One example is the antibiotic

tetracycline.

See alsoProtein synthesis

RRickettsia and rickettsial poxICKETTSIA AND RICKETTSIAL POX

Rickettsiaare a group of bacteriathat cause a number of seri-

ous human diseases, including the spotted fevers and typhus.

Rod- or sphere-shaped, rickettsia lack both flagella (whip-like

organs that allow bacteria to move) and pili (short, flagella-

like projections that help bacteria adhere to host cells).

Specific species of rickettsia include Rickettsia rickettsii,

which causes the dangerous Rocky Mountain spotted fever; R.

akari,which causes the relatively mild rickettsial pox; R.

prowazekii,which causes the serious disease epidemic typhus;

R. typhi,the cause of the more benign endemic or rat typhus;

and R. tsutsugamushi,the cause of scrub typhus.

Rickettsia are transmitted to humans by insects such as

ticks, mites, and chiggers. Usually the insect has acquired the

bacteria from larger animals which they parasitize, such as

rats, mice, and even humans. When an insect infected with

rickettsia bites a human, the bacteria enter the bloodstream.

From there, unlike most other bacteria that cause infection by

adhering to cells, rickettsia enter specific human cells, where

they reproduce. Eventually these host cells lyse (burst open),

releasing more rickettsia into the bloodstream. Most rickettsial

diseases are characterized by fever and a rash. Although all

can be effectively cured with antibiotics, some of the rick-

ettsial diseases, such as epidemic typhus and Rocky Mountain

spotted fever, can be fatal if not treated promptly.

Rocky Mountain spotted fever is one of the most severe

rickettsial diseases. First recognized in the Rocky Mountains,

it has since been found to occur throughout the United States.

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