Microbiology and Immunology

WORLD OF MICROBIOLOGY AND IMMUNOLOGY Lysosome

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negative control of another protein (termed “cro”). In negative

control, the binding of cI to a region of the DNA prevents the

gene from cro from being recognized and used to manufacture

the cro protein.

The “decision” to maintain lysogeny or to begin the

cycle whereby new virus particles are made and the bac-

terium explosively releases the new particles is essentially a

competition between the cI and cro proteins. This competi-

tion centers on the binding of the proteins to a stretch of

DNA called the ORoperator. This stretch of DNA has three

sites that the proteins can occupy. Depending on which sites

are occupied by which protein, the manufacture of either the

cI or the cro proteins is promoted. If more cI is made,

lysogeny continues. If cro is made, the process of viral

assembly (i.e., the lytic cycle) begins. The lytic cycle can be

triggered by events that damage the host bacterium, includ-

ing exposure to environmental stressors (e.g., ultraviolet

radiation exposure).

See alsoBacteriophage and bacteriophage typing; Operon;

Viral genetics; Virus replication

LLysosomeYSOSOME

Lysosomes are small membranous bags of digestive enzymes

found in the cytoplasmof all eukaryotic cells (those with true

nuclei). As the principle site of intracellular digestion, they

contain a variety of enzymes capable of degrading proteins,

nucleic acids, sugars, lipids, and most other ordinary cellular

components. These enzymes hydrolyze (break down) their tar-

get compounds best under acidic conditions. Although lyso-

somes vary considerably in size even within a single cell, the

normal range is usually slightly smaller than the average mito-

chondrion.

The membrane enclosing lysosomes appears to be sim-

ilar to that of other cellular organelles, but it has several

unique properties. First, hydrogen pumps in the membrane

acidify the lysosomal interior to a pHof five, an optimal level

for the activity of its internal enzymes. The membrane has

docking sites on its exterior that allow both materials to be

digested and the enzymes to carry out the job to be transferred

into the lysosome from transport vesicles derived from the

Golgi apparatus, the endoplasmic reticulum, or from endocy-

tosis by the plasma membrane. The lysosomal membrane also

has transport complexes that allow the final products of diges-

tion such as amino acids, simple sugars, salts, and nucleic

acids to be exported back into the cytoplasm, where they can

be either excreted or recycled by the cell into new cellular

components. Finally, by mechanisms that are not yet fully

understood, the lysosomal membrane is able to avoid diges-

tion by the enzymes it contains even though it is composed of

the same compounds that those enzymes routinely destroy.

See alsoCell membrane transport

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