Summary: Expression studies on four members of the pMGA multigene family in Mycoplasma gallisepticumA large family of related genes known as pMGA exists in the avian pathogen Mycoplasma gallisepticum but only a single member of this family was previously found to be expressed in one strain of this bacterium. In this work two unrelated strains of M. gallisepticum were also shown by amino-terminal sequencing to express a unique pMGA polypeptide in both cases. To investigate pMGA gene selection in M. gallisepticum, mRNA expression was analysed in M. gallisepticum strain S6 using reverse transcription-PCR (RT-PCR and Northern blot techniques with probes for several members of the pMGA multigene family. It was shown that the pMGA message is 2·2 kb in size and is monocistronic. RT-PCR detected four different pMGA mRNA molecules but the relative yields were significantly affected by magnesium concentration. By quantitative Northern analysis, the relative abundances of the four pMGA mRNAs in M. gallisepticum S6 total RNA was determined: the pMGA1.1 mRNA predominated [1·88 ng (μg total RNA)−1] but at least three other pMGA genes were found to be transcribed but at much lower levels (20 to 40-fold lower). The pMGA1.1 mRNA is expressed at a level five times higher than the tuf gene known to be one of the most abundantly expressed proteins in the prokaryoti cell. The start point of transcription for pMGA1.1 was determined and probable promoter assigned. From these data it appears likely that transcriptional control plays a major role in the selection of pMGA gene expression in the M. gallisepticum cell. S6