The halophilic archaeon Haloferax mediterranei is a leading microbial platform for producing poly(3-hydroxybutyrate-co-3-hydroxyvalerate) (PHBV), a biodegradable copolyester whose material properties depend on its 3-hydroxyvalerate (3HV) fraction. Besides its well-characterized canonical class III polyhydroxyalkanoate (PHA) synthase PhaEC, its genome encodes three additional phaC paralogs (phaC1, phaC2, phaC3) that were long considered cryptic. Recent work demonstrated that all three paralogs are transcriptionally active, differentially expressed with significant stationary-phase upregulation of phaC1 and phaC3, and regulated independently of polymer amount or composition, and that the truncated, inactive PhaC2 is unique to H. mediterranei among 225 surveyed Halobacteriales genomes. Nevertheless, the native physiological functions of the paralogs, the regulatory mechanism behind their growth-phase-dependent expression (only direct repression of phaC1 by the PspR regulator is known), and the possible post-transcriptional role of phaC1's approximately 350-nucleotide 5' untranslated region all remain unresolved.

Background

Haloferax mediterranei is a halophilic archaeon — a member of the haloarchaea (the order Halobacteriales), microorganisms that require near-saturated salt concentrations for growth — and one of the most intensively studied microbial producers of polyhydroxyalkanoates (PHAs). PHAs are linear polyesters of hydroxyalkanoate monomers that many microorganisms accumulate as intracellular carbon and energy reserves and that are pursued industrially as biodegradable plastics. H. mediterranei naturally synthesizes poly(3-hydroxybutyrate-co-3-hydroxyvalerate) (PHBV), a copolymer whose 3-hydroxyvalerate (3HV) fraction governs flexibility and other material properties, from inexpensive unrelated carbon sources; its high-salt physiology simplifies contamination control and polymer recovery.

The key polymerizing enzyme is a class III PHA synthase: a two-subunit enzyme in which the PhaC subunit carries the catalytic Cys-Asp-His triad that ligates (R)-3-hydroxyacyl-CoA monomers into the growing polyester chain, while the smaller PhaE subunit is required for activity in vivo. In H. mediterranei the phaEC Hme operon encodes this canonical synthase; its deletion abolishes PHBV synthesis, and it sits in a four-gene cluster (phaR-phaP-phaE-phaC) whose transcription is controlled by PhaR, a DNA-binding repressor that also governs the phasin PhaP (a granule-associated protein). Genome sequencing revealed three additional phaC paralogs — homologous gene copies derived from a common ancestral gene — scattered outside this cluster: phaC1 on the chromosome, phaC2 and phaC3 on a megaplasmid (a large extrachromosomal replicon). Because reverse-transcription PCR (RT-PCR, which detects whether a gene is transcribed into RNA) found transcripts only for phaC Hme during exponential growth, these paralogs were long labeled 'cryptic'. Yet heterologous expression in the related haloarchaeon Haloarcula hispanica showed that PhaC1 and PhaC3 can each form an active synthase with PhaE Hme, producing PHBV with distinct 3HV fractions and thermal properties, whereas PhaC2, which lacks the C-terminal region indispensable for activity, is inactive.

Recent quantitative RT-PCR work (measuring relative transcript levels across conditions) overturned the cryptic-gene picture: all three paralogs are transcribed, at levels far below phaC Hme but with pronounced upregulation of phaC1 and phaC3 in stationary phase, and this expression pattern is independent of polymer amount or monomer composition. A survey of 225 Halobacteriales genomes found that roughly 17% of species encode multiple PhaC paralogs (at most two or three, predominantly PhaC1-like or PhaC3-like), and no species other than H. mediterranei encodes a PhaC2 homolog.

Multiple phaC paralogs also occur in bacteria, with divergent outcomes: in Rhodospirillum rubrum the three paralogs divide labor, the most abundant one dominating production and incorporating distinct monomers; in Bradyrhizobium japonicum five paralogs are annotated, only two are significantly expressed, and their knockouts have opposing effects on PHA accumulation. In H. mediterranei, one regulator of the paralogs beyond PhaR is known: PspR, a phosphoenolpyruvate-synthase-like protein (an enzyme homolog that evolutionarily converted into a DNA-binding repressor) whose gene lies in a divergent operon with phaC1. Deleting pspR de-represses phaC1 and phaC3 (Chen et al. 2020, measuring in an engineered strain also lacking the canonical phaC, reported upregulation of all three paralog transcripts) and, in a strain lacking the canonical phaC, restores PHBV synthesis through PhaC1; PspR directly binds and represses the promoters of phaR and phaC1. Even with this progress, the central questions remain: what the paralogs actually do for the cell under their native growth-phase-dependent regulation, how that regulation is achieved (PhaR does not appear responsible, and motif-discovery analysis of the paralog promoter/operator regions found no conserved motifs, including no PhaR binding site), and whether the unusually long (~350 nt) 5' untranslated region (5'UTR, the mRNA segment upstream of the coding sequence, which can modulate translation efficiency and mRNA stability) on phaC1 adds a post-transcriptional regulatory layer.

Problem Statement

Establish the native roles and regulation of the three paralogous PHA synthase catalytic-subunit genes phaC1, phaC2 and phaC3 in polyhydroxyalkanoate metabolism of Haloferax mediterranei. Because PhaC2 has shown no polymer-synthesizing activity in any assay performed, the native-function strand below concerns PhaC1 and PhaC3, the two paralogs demonstrated to be capable of synthesizing polymer, while phaC2 enters through the regulatory strand, which spans all three paralogs. These three strands are all required for resolution and jointly define one objective — moving from the observation that the paralogs are transcribed and growth-phase-regulated to a mechanistic account of what they do and how they are controlled — and comprise: (1) Native function: determine what PhaC1 and PhaC3 contribute to PHA metabolism in the wild-type strain under their native growth-phase-dependent expression — in particular whether they measurably affect the properties of the synthesized polymer (for example the PHBV 3HV fraction, molecular weight or thermal behavior) or participate in stress responses, as suggested by their stationary-phase upregulation; evidence must come from the native context, since heterologous expression and engineered de-repression already show the proteins can function. (2) Regulatory mechanism: identify the trans-acting factors and cis-acting elements that produce the growth-phase-dependent regulation of the paralogs — PhaR does not appear responsible, motif-discovery analysis found no conserved promoter motifs, and PspR's direct repression of phaC1 does not by itself explain stationary-phase upregulation or the regulation of phaC3 and phaC2. (3) Post-transcriptional layer: test whether the ~350-nucleotide 5'UTR of phaC1, absent from phaC Hme and the other paralogs, regulates its translation efficiency or mRNA stability.

References

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