We report the identification of the first histone-like protein of Mycobacterium 
tuberculosis (MTB) (HLPMt). The T cell blot assay was used to identify antigens 
of MTB associated with human immune response in healthy contacts. Fraction 21 
corresponding to proteins in the molecular weight range of approximately 30 kDa 
were found to be immunogenic in tuberculin reactors. None of the fractions were 
found to be immunogenic by this assay in non-reactors to tuberculin. All sera, 
irrespective of the source, showed reactivity with MTB antigen(s) over a wide 
molecular weight range (205-->16 kDa). In the present study fraction 21 was 
processed for the generation of murine polyclonal sera and amino acid 
sequencing. The sequence of a 16-amino acid long peptide showed a 100% homology 
with an open reading frame (ORF) in the translated sequence of cosmid cY349 
(Sanger Centre, Cambridge, UK). The ORF was predicted to code for a protein of 
214 amino acids. Oligonucleotide primers were synthesized based on the 
nucleotide sequence located at the 5' and 3' regions of the gene. The gene 
encoding the predicted protein was PCR-amplified, cloned, sequenced and 
expressed in Escherichia coli as a protein of 28 kDa. The expressed HLPMt 
protein was shown to react with the polyclonal murine sera originally raised 
against fraction 21. Human immune response to the recombinant HLPMt protein was 
demonstrated by its ability to induce lymphoproliferation in peripheral blood 
derived mononuclear cells, and the presence of anti-HLPMt antibodies in pooled 
patient sera by immunoblot. The recombinant HLPMt protein elicited a vigorous 
lymphoproliferative response especially in healthy tuberculin reactors compared 
to non-reactors and patients of tuberculosis, (P < 0.05). The protein has unique 
dual domains with homology to both bacterial histone-like proteins (HU) and 
eukaryotic histone H1. Homology to prokaryotic and eukaryotic deoxyribonucleic 
acid (DNA)-binding proteins suggested that HLPMt could bind DNA. DNA-binding 
properties were confirmed by South-Western analysis strongly suggesting an 
interaction between HLPMt and the MTB chromosome.