Recently, a sensational study on endospore formation in Mycobacterium

marinum has been published (Ghosh et al., 2009); however, this claim was not confirmed in a later study (Traag et al., 2010). According to WHO, find more one-third of the world’s population is latently infected with Mycobacterium tuberculosis (MTB) (Inge & Wilson, 2008), which likely persist as dormant cells in the human organisms, posing a significant problem due to resistance to chemotherapy (Mitchison, 1980). Although dormancy is the commonly accepted explanation of latent mycobacterial infection (Young et al., 2005), limited information has been available about persisting bacterial forms and molecular mechanisms behind their stability and resistance to stressful factors. Among the known mechanisms responsible for the adoption of stress resistance MG-132 datasheet of bacterial cells, it is worth considering the role of histone-like proteins, which bind DNA, changing its topology (Dorman & Deighan, 2003)

and making it more stable against damage caused, for example, by γ or UV radiation (Boubrik & Rouvière-Yaniv, 1995). In Escherichia coli, histone-like proteins HU, H-NS, FIS also play an important role in transcription, recombination and replication (Thanbichler et al., 2005 and references therein). Histone-like protein, Hlp, is present in Mycobacterium smegmatis and contains the N-terminal domain, homologous to HU and the C-terminal domain with the mycobacterial specific PAKKA motif (Mukherjee et al., 2008). Regarding the physiological function of Hlp, it is worthwhile to note the significant increase in its level during transition of M. smegmatis cells to a nonreplicating state under microaerophilic conditions in the Wayne dormancy model. However, the viability

of cells of M. smegmatis strain with inactivated hlp gene was not clearly distinct from that of wild-type strain (Lee et al., 1998) in the same dormancy model. We may reason that Hlp has no significant RANTES role in the transition to dormancy in the relatively short-term Wayne model but may be essential for developing dormancy in nonreplicating cells at later stages. Indeed, many genes, different from those expressed in cells undergoing starvation in the Wayne model, are upregulated at late stages (>24 h) in M. tuberculosis cells subjected to hypoxia (enduring response) (Rustad et al., 2008). The objective of the present study is to clarify the role of Hlp in dormancy in M. smegmatis cells obtained in two experimental models after incubation in a prolonged stationary phase. We found that Hlp was essential for survival of NC cells or for a greater stability of specialized dormant forms, likely due to DNA condensation. Strains and plasmids used in this study are listed in Table 1. Mycobacterium smegmatis strain MC2 155 was routinely maintained on solid (1.