Genes encoding the Mycoplasma arthritidis surface-exposed lipoprotein MAA1 were cloned and sequenced from MAA1-expressing strains 158p10p9 and PG6, from a low-adherence (LA) variant derived from 158p10p9 that expresses a truncated version of MAA1 (MAA1Delta) and from two MAA1-negative strains, 158 and H39. The deduced amino acid sequences of maa1 from 158p10p9 and PG6 predicted, respectively, 86.5- and 86.4-kDa basic, largely hydrophilic lipoproteins with 29-amino-acid signal peptides and predicted cleavage sites for signal peptidase II (Ala-Ala-Ala downward arrowCys). The truncation in the LA variant resulted from a G-->T substitution at nucleotide 695, which created a premature stop codon. This, in turn, generated a predicted 26.6-kDa prolipoprotein (23.6 kDa after processing), consistent with an M(r) of approximately 24,000 calculated for MAA1Delta. Similarly, absence of MAA1 expression in H39 and 158 resulted from C-->A substitutions at nucleotide 208, generating premature stop codons at that site in both strains.