To estimate the minimal gene set required to sustain bacterial life in nutritious conditions, we carried out a systematic inactivation of Bacillus subtilis genes. Among Ϸ4,100 genes of the organism, only 192 were shown to be indispensable by this or previous work. Another 79 genes were predicted to be essential. The vast majority of essential genes were categorized in relatively few domains of cell metabolism, with about half involved in information processing, one-fifth involved in the synthesis of cell envelope and the determination of cell shape and division, and one-tenth related to cell energetics. Only 4% of essential genes encode unknown functions. Most essential genes are present throughout a wide range of Bacteria, and almost 70% can also be found in Archaea and Eucarya. However, essential genes related to cell envelope, shape, division, and respiration tend to be lost from bacteria with small genomes. Unexpectedly, most genes involved in the Embden-Meyerhof-Parnas pathway are essential. Identification of unknown and unexpected essential genes opens research avenues to better understanding of processes that sustain bacterial life.

Bacteria need dedicated systems that allow appropriate adaptation to the perpetual changes in their environments. In Bacillus subtilis, two HtrA-like proteases, HtrA and HtrB, play critical roles in the cellular response to secretion and heat stresses. Transcription of these genes is induced by the high-level production of a secreted protein or by a temperature upshift. The CssR-CssS two-component regulatory system plays an essential role in this transcriptional activation. Transcription of the cssRS operon is autoregulated and can be induced by secretion stress, by the absence of either HtrA or HtrB, and by heat stress in a HtrA null mutant strain. Two start sites are used for cssRS transcription, only one of which is responsive to heat and secretion stress. The divergently transcribed htrB and cssRS genes share a regulatory region through which their secretion and heat stress-induced expression is linked. This study shows that CssRS-regulated genes represent a novel class of heat-inducible genes, which is referred to as class V and currently includes two genes: htrA and htrB.Since the conditions in natural environments are highly variable and unpredictable, eubacteria need systems that support their adaptation to perpetual changes. For this purpose, relevant stimuli must be sensed and identified by the cells and subsequently the resulting information must be transformed into appropriate transcriptional or behavioral responses.In many cases, two-component systems are used for signal transduction (18,24). The presence of multiple two-component systems is a prerequisite to adequately respond to diverse stimuli received by the cells. Such systems consist of a sensor histidine kinase and a cognate response regulator. In many cases, the sensor kinase is located in the cytoplasmic membrane. It is responsible for sensing environmental or nutritional stimuli and transferring this information to the second protein of the system through autophosphorylation and phosphotransfer reactions. When the cognate response regulator is phosphorylated, it either activates or represses the transcription of specific genes, thereby eliciting a cellular response appropriate to the original stimuli.