The genes identified as essential for acid tolerance (Table 2) are probably only a small proportion of the total required, if the frequency of mutation to acid sensitivity correctly suggests 20-50 genes. The gene functions so far identified are quite diverse, and the lack of apparent linkage between them poses obvious problems for genetic transfer of acid tolerance from strain to strain. Thus far, all genes for acid tolerance appear to be present in all strains, raising the question of what subtleties of function or control are present in the acid-tolerant strains.

The connection between acid- and metal-tolerance may mean that for the root nodule bacteria the toxic effects of low pH may involve components of both proton excess and heavy metal toxicity.

While the actS-actR system is constitutively expressed and essential for acid tolerance, it is also clearly a global regulator akin to regulators in photosynthetic bacteria. What it perceives (pH, redox state, PO2), where the signal is received, and how it passes from ActS toActR and onward to regulate gene expression, remain to be discovered.

In S. meliloti WSM419, it seems likely that there are two other systems responding to low pH - the phrR system and that controlling IpiA. The specificity of the low pH induction of IpiA makes it the more interesting in relation to low pH control, but that involving phrR is presumably part of the system integrating other cellular stress responses with that to low pH.

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