The moderately halophilic archaeon Haloferax volcanii was surveyed for protein profile changes correlated with growth at high and low salinity. A single polypeptide with an approximate mass of 46 kDa was conspicuously more abundant during growth at high salinity. This protein was identified as HMG-CoA reductase (HMGR), encoded by the hmgR gene. HMGR is a key enzyme in the mevalonate pathway of isoprenoid biosynthesis, the sole route in haloarchaea for lipid and carotenoid production. Enzymatic assays confirmed that HMGR activity is more abundant in cells grown at high salinity. Low salt cultures of H. volcanii contained lower amounts of hmgR transcript compared to cells grown in high salt suggesting that the observed regulation occurs at the level of transcription. Paradoxically, both lipid and carotenoid content decreased in H. volcanii grown at high salinity despite the increased levels of HMGR specific activity. To our knowledge, this is the first report demonstrating that the expression of HMGR is regulated in response to non-optimal salinity in a halophilic archaeon.

SUMMARYThe B lymphocyte subsets of X-chromosome-linked immune-de®cient (XID) mice were examined by¯ow cytometric analyses of spleen and peritoneal cells. As shown in prior studies, young adult XID mice had reduced representation of the CD5 (B-1a) subset in their peritoneal cavity. However, the CD11b (B-1b) B-cell subset was present and exhibited the IgM hi CD45 lo CD23À phenotype characteristic of most B-1 cells. Although present at a lower frequency than that found in their normal counterparts, B-1b cells were evident in CBA/N and (XD2J)F 1 male mice. With increasing age, B-1b cell number increased and in the oldest XID mice were present as B-cell chronic lymphocytic leukaemia. These results show that XID mice do have B-1 cells, particularly the B-1b subset.

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