Although mycoplasmas lack cell walls, they are in many respects simila
r to the gram-positive bacteria with which they share a common ancesto
r. The molecular biology of mycoplasmas is intriguing because the chro
mosome is uniquely small (< 600 kb in some species) and extremely A-T
rich (as high as 75 mol% in some species). Perhaps to accommodate DNA
with a lower G + C content, most mycoplasmas do not have the ''univers
al'' genetic code. In these species, TGA is not a stop codon; instead
it encodes tryptophan at a frequency 10 times greater than TGG, the us
ual codon for this amino acid. Because of the presence of TGA codons,
the translation of mycoplasmal proteins terminates prematurely when cl
oned genes are expressed in other eubacteria, such as Escherichia coli
. Many mycoplasmas possess strikingly dynamic chromosomes in which hig
h-frequency changes result from errors in DNA repair or replication an
d from highly active recombination systems. Often, high-frequency chan
ges in the mycoplasmal chromosome are associated with antigenic and ph
ase variation, which regulate the production of factors critical to di
sease pathogenesis.