A gene encoding the 33-kDa secreted protein of Mycobacterium tuberculosis (antigen 85-C) was isolated and sequenced. The corresponding DNA sequence contains a 1,020-bp coding region. The deduced amino acid sequence corresponds to a 340-residue protein consisting of a 46-amino-acid signal peptide and a 294-aminoacid mature protein. Comparison with previously described genes for the 30-kDa antigen (the a antigen of M. bovis BCG, also called antigen 85-B) and the 32-kDa antigens from M. bovis BCG and M. tuberculosis (antigens 85-A) indicates that the three genes share considerable sequence homology (70.8 to 77.5%) but may also code for distinctive epitopes. Strong differences among the three sequences are clearly visible upstream and downstream from the region coding for the mature proteins. The three genes have been detected in the genome of M. bovis BCG by Southern blot hybridization with three type-specific probes. Furthermore, hybridization of large DNA fragments (100 to 1,000 kbp) from M. tuberculosis separated by pulsed-field gel electrophoresis showed that the three genes coding for the antigen 85 complex are not clustered within the bacterial genome.

Proteins of the antigen 85 complex are abundantly secreted into the culture supematant of a variety of mycobacteria (1, 2). These proteins are known to be responsible for the high affinity of mycobacteria to fibronectin (3). The 32 kDa protein (antigen 85A) appears to be a major stimulant of cellular and humoral immunity towards mycobacteria both in mice and man (4-6). The gene encoding the 32 kDa protein of Mycobacterium tuberculosis has recently been identified by us (7).Comparison of this gene with the a-antigen from M. bovis BCG (Tokyo) (also referred to as antigen 85B or as MPB 59 protein (8)) revealed 73.8% homology between coding regions of the DNA sequences (7, 9). This figure should be corrected to 77.5% considering that both sequences have been revised since their publication (10, and this report). It is of interest that the homology is limited to the region coding for the mature protein and the carboxy-terminal part of the signal sequences.After screening a Xgtl 1 BCG library (prepared from M. bovis BCG strain 1173P2) with a 5'-230 bp PstI fragment from our M. tuberculosis 32 kDa protein gene (7) (see Figure 2)